How do you fix a spongy floor on a boat

I assume it is a wooden boat/floor .... odd location for this question .... but whatever! If the floor is part of the boats hull, then take the boat out of the water ... allow it to dry completely. Check the wood with a knife/screwdriver etc.. to see if it is rotted/flaky. If so, replace the wood. If no, you can fiberglass the wood on the underside and give it a weatherproof coating on the topside. There is a dampness issue so check the balance of the hull for any concentrations of dampness that need attention. Added comment- Above answer is good, but to clarify if floor is spongy you must remove & replace old plywood deck, usually in entirety but some have successfully spliced in only a part. All is better because you know what you have underneath that way. Check stringers below deck, if rotted or softened much you must replace or boat will be unsafe. Check transom, if it is wet or has rot you'll have a far larger project than just the deck to contend with. Remove old flotation foam if present because it will be saturated with water if the deck has this problem. If any white fungus is present paint with antifreeze to kill it, then wait a few weeks before waterproofing wood with the same polyester resin/hardener you'll use to seal both sides of the deck plywood & use for glassing in stringers & deck top surface. Use stainless steel wood screws when attaching new deck to stringers, then glass the surface after that. There are a number of good web pages boat owners have chronicled their deck repairs in, worth looking at.