You don't do insurance claims in the store. You do them over the phone. If you're with Verizon, you probably have Asurion for your insurance provider. Their number is 800-641-0019. I'd also just tell them you accidentally dropped it in the sink. They won't replace it unless it's "accidental user damage." You won't be able to get an 8300 either. The insurance will replace your phone with a refurbished phone of the same model.
If you're expecting the store to replace your phone under warranty, it won't happen. The manufacturer's warranty covers manufacturer's defects. You taking your phone into the ocean is not a manufacturer's defect. It's user damage. Don't try to tell the store that the phone has "never been wet, and I don't know why you would think that," either. That doesn't work. Actually, you'll just irritate the person you're talking to.