Weird starting issue, need help

So I have a really weird starting issue. My boat won't start in the water, acts like the batteries are dead. Pull it out and it will fire up. Now I assume this has something to do with back preasure some how.....Now things I tested for. Batteries are all good. Charging system all good. When its in my driveway hooked up to Fake a Lake it fires right up. But as soon as I dip it seems to barely want to crank. I'm lost to even know where to start on this. I assumed it was the batteries or alt but both checked out fine when tested. thanks