There is no reason you should let a dead battery beach your boating plans. If you can't afford a good marine battery right now just run to Autozone and grab a $40 special, it won't last very long but it will get you out a few times. If thats a problem steal the battery out of your car/truck.
I had to do this yeas ago, launched the boat with a dead battery. I had a choice of loading the boat back and going home or searching for a battery or parking the truck and borrowing the battery out of the truck. I was on the water all day with no problems.