My boat won't start

Any ideas to help me out before i take it in?

I've been pulling my boat out of the garage every three weeks or so and running it with the fake-a-lake during the winter months. Today, i went out there to start it, and all i get is a click from the engine compartment. I'm assuming the batteries are ok, as i have two of them, and they seem to be firing up the stereo just fine, but let me know if there is anything i should check before bringing it in to the dealer. Thanks.