Just got caught in major rain, no power after I turned car off

The battery connections could be loose, clean and tighten them. The ground cable is next on the list to check. Clean and retighten that. Take a volt meter and check the voltage from the hot (+) on the battery to the frame/engine, it should read 13.8 on a full charge. If you get nothing check across the battery terminals with the volt meter, again it should be 12.8 to 13.8 volts. If there is no voltage , you're battery is dead, shot, kaput. One more test, with the head lights on go from the negative battery post to the engine block. That should read zero voltage. If you read anything you have a bad ground at the battery. I see you have the battery in the trunk, just go from the (-) post to the car frame. I ran across a car at a friends shop that was dead, battery was perfect. The owner detailed the engine compartment, even painted the battery posts gray. Gray paint will not conduct! The other weak point is the fusible link, look for a weak/bad connection at either end of that 14 gauge link at the bulk head connector. Good luck!!