Looking for some sort of storage unit to fit under the seat in a 2005 Boston Whaler Sport 130.?

Whaler Sport 130. I would suggest getting a dry bag at a marine store and taking it on and off the boat. There is not much hull to drill into, plus the layout is tricky. Usually the best bet (besides building a deck) is getting a removable waterproof bag and fitting it under the seats. I know its kind of inconvienent, but it will work just fine.