Happens on lots of boats. The PO of our boat cracked our settee too by overfilling the tank and blowing it up like a balloon. The tank overflow / vent simply can't keep up with 100 PSI street pressure. Good news is the tank survived just fine and the crack is under the settee cushion and is cosmetic only so no big deal..Is your model a wing keel? fin? bulb?
Filling the water tank too quickly? Really? Never heard of that.