I've got a Motorguide Pro 70 24/volt trolling motor going on it's 4th season. At the beginning of last year, I'd get about 4 hours of use and the power would be nearly gone. Barely move in top speed. Tested the batteries (load tester) and sure enough, one of the batteries was bad. Replaced it and all was well. This year it's starting to act the same way so I figured the other battery needed replacing. Load tested both batteries again and this time they test out fine. I  could take my boat back where I bought it but they're so busy I wouldn't see it for a month. Anyone suggest what else I can check. By the way the connections are clean and tight and there is no fishing line behind the prop. Thanks in advance for your help.

Did you check the connections on the male and female plugs at the front of the boat.  You could have corrosion there.  Make sure the screws that hold the wires are tight as well.  

Are you blowing fuses or popping circuit breakers? Maybe you have a trolling motor with a bad bearing That's pulling alot of current.

Bill

What series are the batteries?   How many reserve amps do they have?