Re: Where exactly is the water or squish?
We have an older 1994 26 and the sole in the main cabin just outside the head door, as well as the head sole, both have always felt the way you describe. I wonder if it is possible that you might just have never noticed the 'softness' and now are sensitized to it. In our case, the sole feels 'bouncy' (like it is on a waterbed) but is just springy rather than sitting on water. It concerned me a lot when first considering the purchase, but several reference checks with other owners, including Crazy Dave, convinced me that the 'softness' is just a design artifact.
Anyway, we have never had an issue with the sole and, if you find no volume of water under it, its probably not worth worrying about. If you do find water, the most likely sources are: 1. an unsealed keel bracket bolt in the CB trunk, 2. rainwater leaking by the teak stops on the sliiding hatch and drifting into the bilge through the tabbing under the galley, or 3. water back siphoning through the bilge pump hose when burying the starboard windows on a tack. Ballast tank leaks are extremely rare.
We did experience water under the cabin sole teak/holly veneer (as BobSail outlined) from the sliding hatch leak but both issues were easily resolved. I suppose its also possible that if you have water under the teak, and you are getting the 'normal' bounce in the head, you might be flexing the main cabin sole and hear the 'squish' from under the teak.