I get the impression that the Enterprise needed to be close to the volcano in order to stay in contact with Spock/the shuttle over the interference from the impending eruption. But they couldn't hover above it because they'd be seen. Hence the ocean. It's a reach, but I think they did try to explain it. (The real absurdity there is the "cold fusion device" -- not only does cold fusion have nothing to do with insta-freezing things, but solidifying the lava on top of a volcanic eruption would actually make things far worse by increasing the pressure of the lava underneath and causing a far bigger explosion.)