[Kon-Peki]

Maybe it is drained to a holding tank down near the keel to add stability. And when at anchor, they pump it back into the pool.

[tfandango]

I shouldn't mention this probably but I watch Below Deck on the Bravo channel which follows the crews of charter yachts. One of which is a sailboat with a hot tub. I think you are right, when it's sailing (which seems to be rather infrequently), the hot tub is drained somehow.