Stomp around on the floor with boots on or use a hammer over the entire floor looking for some spots.
Where you can get to the stringers/bearers (usually in the liftout floor sections if fitted and in the engine bay area) tap them with a hammer also looking for softness.
Have a good look at all the wiring and depending on how long it was submerged you may see a tide line on timber door paneling etc
Is the sterndrive no good also? Or is it shaft drive?
Are you capable of doing the work yourself? What size engine did it have? A single engine 26ft flybridge would want absolutely no less than a 7.4ltr Chev/Mercruiser and even that would be underpowered.
If you repowered that with new you will be up for $25K for engine and drive unit plus fitting plus steering if it is buggered, plus, plus, plus.
Projects like this are only worth attempting if you have the skills and contacts to get what you need at the right price.
Many people embark on these projects only to discover its a money pit and after they have put plenty in they sell off cheaply and lose bigtime.
I recently looked at a Savage 28 flybridge without engines. Was going to buy for 10K and keep for myself. By the time I repowered it, replaced the velvet drives, new shafts, rudders and props I was around 45K. Thats before painting the hull, retrimming and any fitout and canopy work. Not worth it. And I am in the trade...