In the Korean War, the US outflanked much of the North Korean Army by launching a naval invasion at Inchon. This allowed them to advance towards the Chinese Border, and likely would have won the war had the Chinese not intervened. Given that Vietnam also had a long coastline, why did the US not navally invade to outflank the NVA forces, or even attack the North itself from the sea? What factors made this impossible?
Primarily because the US strategy for the Vietnam War was a defensive one.
I'll refer for the sake of ease to this post where I explain why attacking North Vietnam was never within consideration for the US.
I would also like to to point out that a Naval Operation would have meant an Amphibious and rather messy landing (hypothetically) considering that after 1965 aerial bombing was fairly regular (Operations Rolling Thunder, Linebacker, Linebacker II) and within North Vietnam there was a race between the North Vietnamese and Chinese Engineers in repairing the frequently destroyed infrastructure and the US aerial bombings in destroying roads, power plants, and factories.
I don't think though that an offensive invasion was ever really on the cards for the US. Until Nixon, escalation was not ever a consideration for the South Vietnamese or the US high command. And even if there was attempts at expanding the South Vietnamese military capability like Vietnamization, the focus was on removing the impact and presence of US soldiers committed to the war. Considering that the South Vietnamese military still relied on US assistance Vietnamization did little to encourage or solidify any offensive ambitions on the behalf of the South Vietnamese.
As an aside I'll mention that an Amphibious attack (while possible and very effective) would have been incredibly costly and difficult to coordinate considering the layout of the Haiphong Harbor and city (one of the most direct routes to Hanoi). The other problem is that an amphibious attack would have required even more of a commitment of troops from the US. Considering that the NVA would have ample time to see an amphibious assault happen (not to mention that to keep it from being seen massive civilian casualties would have happened), an outflank by attack the North via a sea attack would've been absurdly dangerous as the attackers would have to contend with Chinese Air power, Vietnamese local defense, and AA as well as artillery barrages before needing to land and displacing the defenders.
Considering that May 1972 saw the mining of Haiphong Harbor with Operation Pocket Money, an amphibious assault at any time after (though short it was) would have been physically impossible. Any time after the Tet Offensive (1968) would have required an even larger commitment of resources and troops (the US personnel peaking in April of 1969 at almost 550,000 troops) and any time before the Tet Offensive is a wash considering the internal political turmoil within South Vietnam up until 1965 where the Young Turks (a group of officers consisting of Ngyuen Cao Ky of the VAF, Nguyn Chanh Thi of the I Corps of the AVRN, and Admiral Chung Tan Cang) ended the rotating door of leaders in South Vietnam since Diem.
To further worsen the problem, a startling amount of communist infiltration within the South Vietnamese government would have hampered any prolonged coordination between the South Vietnamese and US militaries. The lack of intelligence and in many cases understanding of local intelligence by the CIA would have made any flanking invasion operation grossly tricky.
Sources:
Shaplen, Robert. The Lost Revolution: The U.S In Vietnam: 1946-1966. Harper & Row (1966)
Moyar, Mark. Triumph Forsaken: The Vietnam War, 1954-1965. Cambridge University Press. (March 2, 2009)
Topmiller, Robert J. The Lotus Unleashed: The Buddhist Peace Movement in South Vietnam, 1964-1966. University Press of Kentucky (Nov, 1 2006)
Cao Ky, Nguyen. How We Lost the Vietnam War. Cooper Square Press (Sept 30, 2002)
Note that there's a few more sources, like the ones mentioned in the link, but I figured that these would suffice with regards to the political side and the linked comment would have the military considerations.