Yes, I didnt see that till afterwards, but the answer is still the same for all the zillions of others questioning it. RAMA
I was getting all ready to make a snarky remark about mercury or snake venom or lava before I realized the OP wasn't being serious.
WHAT?! THE OP ISN'T SERIOUS?! HE IS MAKING FUN OF THE HOLY TREK?! SHUN THE UNBELIEVER! SHUN! SHUUUUNNN!!!
I don't know, but Voyager established that in the Star Trek Universe, starships can be relatively easily modified to go miles and miles underwater.
I don't have an issue with water - the ship survived the pressure of a black hole without even slightly buckling its pylons, water pressure is nothing by comparison. But actually, now that I think about it, two wrongs don't necessarily make a right... I suppose we have to assume that the structural integrity field holds the ship together at the limits of pressure. I do however have an issue with a ship of that shape and size being able to take off and land on a planet though. They haven't set down on land - which would look very silly (although even by TMP, the saucer section was designed with the intention that it could detach and land). Surely the energy required to escape planetary gravity for something of that size and density is going to do a lot of damage to marine life? Massacreing plant and animal life for miles around isn't really Starfleet.
Considering that either they succeed in the mission or the whole planet is dead anyway from what we've seen, some marine life can probably be sacrificed.
Yet there was a Sub Commander Tal in "The Enterprise Incident." Talk about being out of one's element. "Take us to periscope depth!" "Sir?"