I've had great results with Everstart Maxx Marine from Wal-Mart. I have two on my boat; one dedicated to starting, the other for trolling motor, radio, submersible lights, etc.
My batteries are connected to a selectable battery switch . Battery 1/ both/ off. I start and run the boat on both to charge them, then switch to battery 1 for trolling and fishing. That way battery 2 is always fully charged for starting the motor.