When you use the speakers in your car, they do not sap the battery of power because the battery is hooked up to an alternator. This is a device that spins with the engine, using this motion to create power. That power then recharges the battery as you go. That is why you never have to stop and recharge your battery unless you leave the lights on and it drains the power since the engine is not running.
If you get wakeboard speakers for your boat, though, it is important to note that this is not how the speakers are going to work with most boats. The batteries and the engines are independent of one another. Even if you are cursing around the lake all day, the battery is going to be drained by the speakers. This does not mean that you should not use them, but you should charge your battery in preparation for a trip to the lake.
However, it is very important to note that many new, inboard motors – and some modern outboard motors – do come with alternators. They recharge the batteries just like a car. It is mostly older outboard motors that do not utilize this technology. If you are not sure how your boat works, just take a few minutes to glance through the owner’s manual. You may find that an alternator has been installed on your boat, so you have nothing to worry about, no matter how often you use the speakers.