I really think that you have a little too much time on your hands.
The deck is going to flex because it is fibreglass and unsupported. If you were on a 35 foot boat with a lot of reinforcement there, then you might, and I say might
, want to devote a few minutes to determining if there was any wetness inside.
However - kudos for taking the time to check it out - but I'm pretty sure you can find something a little more important to deal with...