First, sorry to hear about this unfortunate incident. As to what caused this to happen, could be any number of things. Being a brand new boat, a broken or cracked through hull fitting, bad seacock, loose clamp on a hose, incorrect wiring on the bildge pump, something that got passed quality control at the factory. As for the engine weight theroy, as long as the 90 hp is not over the rating on the capacity plate, that shouldn't be an issue.If the power head was submerged, not sure I would still want that motor hanging g on my transom.
This sounds like an insurance issue to start, and during the insurance company's initial inspection, they should be able to determine tha cause. If the dealer gets their hands on the boat first, they might cover things up by fixing the error to cut their losses. Either way,depending on your policy coverage, it will probably just cost you your deductible and maybe the rest of the season. Good Luck, let us k ow how you make out.