> Even with triple redundant relays, how do you know the salt water didn't just wet them all?

The design typically includes a mix of normally open and normally closed switches. If everything failed in the same direction (closed) it wouldn't satisfy the failsafe.

If you're spilling conductive liquid on the board, it's going to blow fuses anyway. It's more likely to short to ground than to short only to the precise path needed to activate.