5what it is good for
This is a water mount that happens to work on land, not a travel mount that happens to swim. On the ground it is slower than walking in decent boots and the jump barely clears anything, but the moment you hit liquid it moves properly and your breath meter stops being a concern at all.
That makes it a fine tool for clearing an ocean floor, running sunken chests or setting up a fishing spot, and a poor one for anything that involves getting somewhere quickly. If a Lava Shark has already come out of a crate for you, use that instead. It is faster and it does not care what the liquid is.
trivia
With a Bottomless Water Bucket in hand you can climb forever, dropping water above your head and swimming up into it. The buff text is lifted from Paul Revere's Ride, the line about one if by land and two if by sea.