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To carry on alatar's addict-analogy, which is fitting, I think Gollum would have been very different when he had the ring. Without it, all he could think of was the ring he was robbed of; every waking hour he would obsess about it, and he wouldn't have built anything unless he thought it could be of immidiate help in his quest to retain it.

With the ring in his possession his addiction would be satisfied, but he was far from happy. The hours must have been endless for him deep in the dungeon, far away from anything remotely plesant. He would have fantasized about getting rid of the ring, leaving the mountain and picking up his life again. Yet, this he could never do. Is it not possible then that he would channel this frustration and boredom into something like building a boat, in order to explore his lake and for convenience? After all, he had a curious mind, entering the dungeon partly in a quest to unravel the secrets at the roots of the mountain.

Edit: A lot of addicts are also very creative artists.

Anyway, there are lots of anachronistic and odd details in the Hobbit. It is simply not written as tightly as Tolkien's other works. Gollum's boat, however, is only slightly odd to me.