My thought is that you have some moister under the floor, I did and had a lot of stress cracks as I called them.
Here it was caused from the water under the deck skin and freezing in the winter, exspan and cracking the deck.
Floor is just glass and gel coat like the outside of the boat with texture mix throwm in for griping.
Then real wat to fix them is to use a dermeal tool and V out the crack, and filling with a filler, I like Fromula 27 but it has to be top coated with something as it will assorb water if not.
After filling you can gel coat again or get some Deck paint, I and a few others got Duraback with texture and it has held up real good!
If you have any large deep cracks then you will need to due the same, but maybe some fiberglass cloth to make it hold toghter and not crack, sand and again cover with gell coat or paint.
Here is a link for that Duraback, it will cost you $100+ but it is great stuff!
http://www.nonslipcoating.com/