No way...cars should be designed to work outside where you such things as sun, water, wind and snow occur.IF OP ends up not paying out-of-pocket, it will be because the dealer goodwills the repair. It is 100% impact related, albeit the impact happened against water. If OP approaches the dealer with a sense of entitlement on this matter, he/she may as well not show up at all. Heck, the description of the accident is a total admission of fault. The destructive force of water is well known (i.e. tsunami against the coast of Japan). Personally, I would just buy the part, any missing/broken fasteners, and install it myself. I hate stealerships more than most, buy right is right. Nobody but the owner or his insurance should be expected to pay for physical damage to the vehicle.
If it broke that easily then the manufacturer must fix it.