Tigerr

Thats not more than a tank of water. And a fair bit of rot, which suggests it has been there for a while. Maybe under previous owner, a winter freeze, popping a plastic fitting, and draining the tank into the bilge. Then, discovered, pipe fixed, and shortly after boat sold. Water left under the floor, because - nobody will know for a year or two. Now your problem not the previous owners. It wont be the first time that's happened. Dont panic. It's a damn nuisance but unlikely to sink. Let it dry out. It may continue to drain down to the lowest point for a bit. Those super absorbent cloths are very useful for mopping it out. You will need to work out the extent of the rot damage though, and replace some flooring. Good idea to put an inspection hole in at the lowest point, for future reference.