Water leaked in trunk, bad 12v battery

I went out to my 2013 Prius this morning. I unlocked it with FOB when I got in, I went to start it and nothing happened. There was no lights on the dash, sounds, or any sign of life at all. So I went to the trunk to check the 12v battery (I had to crawl into the trunk to open it manually). Everything seemed to be hooked up tight. It was then I notice there was water under the battery, there was also water in the spare tire compartment (we had a very heavy rain/thunder storm last night). I disconnected the 12v battery and mopped up all the water. As I was reconnecting the 12v battery I noticed the dash lights come on. So I got in and it started up. To make sure it wasn't a fluke I turned it off, waited and restarted. Everything was working perfectly, but to be on the safe side I took it to Napa to have them test the battery. The guy came out with his machine. My wife shut off the engine and the guy immediately connected the device. It showed strong at 12 or 13 volts. Then there was a click and the went into complete shutdown mode and the volts began to drop rabidly. He told me the battery is bad and gave me a jump start (because it wouldn't start) and I went home to figure out what I was going to do. I called O'Riley auto and they said if I bring the battery in They could tell me if the battery was charged, low, or bad. I brought the battery in and he used two machines to confirm it was dead. He told me to go home, put the battery back in, jump it, and bring the car to him. When I got home and put the battery back in the dash lights came on and I was able to start the car without a jump. When I brought the car to the O'reiley guy he preformed several more tests. He said the battery had 12 volts while it was in the car hooked up. He ran his 'tests' and told me the battery was bad and that I need a new starter. I really don't think this guy knows what he's talking about. So I drove home and disconnected the battery (so that maybe it will have enough juice tomorrow to start). Any idea what's going on? Could it be the water? Or did the battery really just die for no reason? the date on the battery is 5/19. I would really appreciate any help. I don't have much money to pour into diagnosing and fixing. Thanks.

 

We've had the car about two years. The past yearend a half. we have been living in Southern Illinois. I did not buy the battery (unfortunately, so I can't take advantage of the 84 month warrantee on it. I drive the car everyday, even through Covid. The past month I have been using it for Uber--probably putting 15-20 miles on it a day. What were you saying about disconnecting and conecting?