What 21k to replace? I would definitely file an insurance claim rather than bringing up a warranty issue if your car is submerged. Proof of submersion, such as a high-water mark on interior panels, is required by insurance companies. The battery ought to be changed under warranty if the vehicle wasn't submerged and a leak was created by regular driving. Tesla has a reputation for breaking warranties, but after Elon Musk leaves the firm, this will probably stop happening.