Fort Clatsop, where the Corps of Discovery wintered, was not within sight of the Pacific. To get to the ocean required either an overland journey of several miles through steep, thickly forested terrain followed by coastal swamps, or a somewhat risky canoe voyage out into the winter waves of the Columbia River's tidal estuary.

A small group had been tasked with finding a suitable place to boil seawater for salt, but the place they decided on (near today's Seaside, Oregon) was 15 miles from Fort Clatsop. The salt-site task force also reported that local Indians had discovered—several miles further down the coast—a beached whale. Lewis and Clark would not have thought it a good idea for Sacagawea, still a nursing mother, to go on such a sojourn—but they did not bar her when she asked to join a party that was going to the ocean to harvest blubber from the whale.

Here is Lewis's journal entry for January 6, 1806:

"Charbono and his Indian woman were also of the party; the Indian woman was very impo[r]tunate to be permitted to go, and was therefore indulged; she observed that she had traveled a long way with us to see the great waters, and that now that monstrous fish was also to be seen, she thought it very hard she could not be permitted to see either (she had never yet been to the Ocean).

So denying her a view seems the wrong interpretation, as relatively few members of the Corps had yet seen the ocean. The constant rain and fogs made it very difficult to see downriver to the mouth of the Columbia, and it would be difficult to find a vantage point where a human could see over the heavy forests of the area.

According to Lewis and Clark's own journals, Sacagawea did not get to see the ocean when members of the Corps of Discovery first had the opportunity. The most reliable primary sources of the Lewis and Clark Expedition --the journals of its members-- clearly demonstrate that Sacagawea was prohibited from leaving camp to join the men on their first trips to the beach. What can't be known for sure is if she was the last member of their party to reach the ocean, or what the reasons might have been for not allowing her to see the ocean when the group first had the opportunity to do so.

u/MrDowntown is correct about the geography, but wrong about the chronology. Construction of Fort Clatsop began on December 7th, 1805 on the Oregon side of the Columbia, yet the Corps of Discovery had been on the Lower Columbia for a month at that point and were already visiting the ocean. On November 7th, the group reached the villages of the Wahkiakum tribe and first sighted what they thought was the Pacific.

Great joy in camp we are in View of the Ocian this great Pacific Ocean which we been So long anxious to See.

William Clark, Nov. 7, 1805

Ocian in view! O! the joy.

Meriwether Lewis, Nov. 7, 1805

The Corps of Discovery had not actually reached the ocean at this point and were really seeing only the river's confluence with the Pacific ahead. It would take them another week to have that chance. Despite being just a few miles away, heavy rainfall and steep terrain prevented the group from moving. The group erected a makeshift camp at a location Clark called "Dismal Nitch" in his journals, where they were pinned in under the relentless rain for 6 days.

Finally on November 15, the Corps of Discovery was able to reach a more sheltered location a few miles west which they called Station Camp. Station Camp was the site of the middle village of the Chinook, and the travelers remained at the site for ten days to recuperate and trade with the tribes of the area. With Station Camp as their base, the group would finally be able to make day trips to the ocean coast.

We are now in plain view of the Pacific Ocean. the waves rolling, & the surf roaring very loud ... We are now of opinion that we cannot go any further with our Canoes, & think that we are at an end of our Voyage to the Pacific Ocean, and as soon as discoveries necessary are made, that we shall return a short distance up River & provide our selves with Winter Quarters.

Joseph Whitehouse, Nov. 16, 1805

The next day is the day that the group made trips to stand at the edge of the continent and get their first grand uninterrupted view of the ocean.

At half past 1' O Clock Captain Lewis returned haveing traversed Haley's Bay to Cape Disappointment and the Sea Coast to the North for Some distance. ... I directed all men who wished to See more of the main Ocian to prepare themselves to set out with me early on tomorrow morning.

William Clark, Nov. 17, 1805

So what does this tell us? While it is not stated who accompanied Captain Lewis on his morning trip to see the ocean, we can be assured that he did not go alone, as there are no other examples in the Lewis and Clark journals of the captain's venturing off solo. According to Lewis and Clark historian Gary Moulton, 11 men accompanied Captain Clark the following day, including Sacagawea's husband Touissaint Charbonneau. The Corps of Discovery counted 31 men, therefore by December 18, 1805, at least 13 of those men had seen the ocean, and there were almost certainly more who had accompanied Captain Lewis on December 17.

In the period in between their stay at Station Camp and January 7, when Sacagawea finally saw the ocean on a trip to a Nehalem village to see a beached whale, many more members of the Corps of Discovery could potentially have visited the ocean for the first time if they had not seen it already. On December 28, a detachment of five men were sent to the ocean to construct a salt making camp where men would be stationed for days at a time. Among them was George Gibson, who did not take part in Clark's excursion to see the ocean on December 18. Including Gibson among the known ocean-viewers, the number of men who had not seen the ocean could be as few as zero or as many as maybe 12. But, considering that daily hunting trips sometimes took men up to 10 miles away from the fort, it is easily to imagine that by the time Sacagawea got to see the ocean on January 7, every man in the group already had. Without documentation we can't know for sure.

While we can't be certain if Sacagawea was the last member of the group to see the ocean, the journals give a strong indication that she was indeed held back from seeing it as early as she would have liked.

The last evening [Charbonneau] and his Indian woman was very impatient to be permitted to go with me, and was therefore indulged; She observed that She had traveled a long way with us to See the great waters, and that now that monstrous fish was also to be Seen, she thought it verry hard that She Could not be permitted to See either (She had never yet been to the Ocian).

William Clark, January 6, 1806

But why was she held back? Clearly, after traveling with the men from Fort Mandan and through the snows of Lolo Pass she was physically capable for the short walk to the beach from Station Camp. If there was any doubt about this, her trip to see the whale on January 7 proved it, as the group had to hike Neakahanie Mountain to get there (Sacagawea, while carrying her baby Jean Baptiste).

Evidence from the journals suggest social difference as the most likley justification. Much has been made about Sacagawea being allowed to cast a vote while at Station Camp for where the party would make their winter quarters. In Clark's November 24 vote tally though, Sacagawea's name (or more accurately, the name "Janey" which Clark sometimes called her) appears at the bottom of the list, and instead of making a selection between the different options, her vote is listed as "Janey in favor of a place where there is plenty of Pota[toe]s". I say this not to diminish the significance of her participation, but to demonstrate how little the opinions and desires of a teenage Shoshone girl mattered to the men of the expedition.

In fact, just four days before that vote, the captains traded away what was probably Sacagawea's most prized possession in exchange for the otter skin robe of a Chinook chief.

one of the Indians had a roab made of 2 Sea Otter Skins the fur of them were more beautiful than any fur I had ever Seen both Capt. Lewis & my Sel endeavored to purchase the roab with different articles at length we procured it for a belt of blue bees which the Squar --wife of our interpreter [Charbonneau] wore around her waste.

William Clark, Nov. 20, 1805

There is nothing to suggest that they asked for Sacagawea's permission to sell her property for their own gain.

So to summarize, I would contend that sources clearly demonstrate that Sacagawea 1.) was one of the last --if not the last-- members of the Corps of Discovery to see the ocean despite having easy opportunity to do so for nearly two months; 2.) that her not seeing the ocean was deliberate; and 3.) that while the reason she was held back is unclear, the men of the Corps of Discovery no doubt felt they had every right to restrict her movements, probably because of factors like her gender, her age, her race, or a combination of the three.