I've noticed a somewhat common viewpoint lately saying that the USSR was about to invade Japan, and that the US was forced to use the atomic bomb to force Japanese surrender before this could happen.

You've got it a little backwards and it is just worth clarifying. The USA had secured agreement from the USSR at Yalta to declare war on Japan a few months after V-E day. By Potsdam the USA was no longer all that interested in the USSR's involvement in the Pacific front, because it had seen what happens when the Soviets "liberated" territory from the Germans (magically all of the local Communists took power afterwards and started doing exactly what the USSR wanted). The USSR would also get some key geopolitical assets if it entered the war: the rest of Sakhalin and the Kuril islands (which gave it easy access to the Pacific from Vladivostok), and the port of Dalian, in Manchuria.

But the USA didn't feel it could wiggle out of the Yalta arrangement. So the argument is that the US used the bomb as soon as it could (the first good-weather day after Potsdam ended) to maybe end the war before the Soviets could get into it, thus saving the US some later hassle. As it was, the bomb didn't immediately end the war, the Soviets immediately declared war on Japan and pushed their Manchurian invasion up, and ended up with the concessions agreed upon. (And it has been argued that the Soviet invasion played at least as much of a role in the Japanese surrender as the bombs, etc.)

Now could the USSR have invaded Japan by sea? Not immediately, no. Their short-term role would be to take over Japanese holdings in Manchuria, Korea, etc., which were necessary for the island to wage war (raw materials, food, etc.). Amphibious invasion would have been something that would have needed a lot more planning. Even the invasion of the Kurils, which was undertaken in late-August, early-September 1945, was tricky and difficult. Invading the homeland would have been very difficult indeed.

Could the Soviets have invaded Hokkaido if they wanted to? They seem to have thought so. The idea was explicitly floated in June 1945 (and rejected doing it immediately because it was feared it would upset the Americans), and the Japanese seemed to fear it. The distance between Sakhalin and Hokkaido is not very significant, hardly "an ocean" — it is only 25 miles or so. It is not nearly as big an operation as the US invasion of Japan, or even D-Day (it is 1/4th the distance of the English Channel). Which is to say, would it have happened in the next week or two? No. If the Japanese had kept at the war, would it have eventually happened, after the Soviets had secured their hold on Korea, Manchuria, and the islands? Why not? The Soviets thought it would, and the Japanese thought it would. Would it be messy and costly? Sure — but that's exactly why the Japanese feared the Soviets, because they knew that you could not "bleed them out" the way you could the Americans, because Stalin was willing to trade the lives of soldiers for territory.

There are a lot of details on the Soviet plans/Japanese fears in Hasegawa's Racing the Enemy, which is still the best source of information on the Soviet/Japanese aspects of the end of World War II.