The deck fits on the hull like a shoe box, and is made in a separate mold.
So it can crack over time, because it's not an absolute perfect fit. both Glastron's I have owned had a crack similar to the one you showed us.
One boat had a good transom and the other had a transom that pasted the "Try to rip it off" test but still had rot in two places.
The pilot tube that runs down to the speedo pick up tube was not sealed/bedded right or just got old and let water in.
Water soaked down from the three hole and over to hull drain hole, wood was so rotted I removed it with just my fingers.
It would have been OK for years like this because of how my transom is designed, but I knew my boat would be doing over 50 mph and didn't want to risk it, it also needed a repair on rear floor and stringer so I replaced transom and rear floor/stringer.
My boat had a mid 70's Mercury with the thumb screw upper mounts and the old style add on T&T system, so I had holes all over the fiberglass on my transom. I used a "Transom Saver" on the inside of the splash well to cover old mounting holes, I'm sure it also adds strength to mounts, but I did it more as a way to cover holes and large circles from old motor.
I also lifted my motor so T&T ram was up against splash well drain hole, my splash well would still drain ... but very very slowly ...
I found some plates that added more negative trim to your motor and used them to get motor back from drain hole about a quarter inch.
Are theses the thing your talking about ?
T-H Marine transom support plates ...
http://thmarine.com/th-marine-products/outboard-performance/outboard-transom-support-plates.htmlTransom Wedges ...
http://thmarine.com/th-marine-products/outboard-performance/outboard-transom-wedges/cmc-transom-wedges-2-degree.htmlYour motor looks to be a 70's motor, so must have been added at some point, poke around the motor mount bolt holes, look for cracks in fiberglass covering on inside of transom.
It's hard for us to "Make the call" with out actually looking your boat over ...
Wood rot from poorly sealed speedo tube screws ...
Transom saver ..
Negative trim plates ...