Most speakers will come with their own clamps and fasteners for you to attach them to your boat. Make sure to choose a set of speakers some with adjustable clamps and waterproof clamps. You do not want your fasteners to break down and come loose.
When you are positioning your speakers, the height and angle are important. You want your speakers to be facing the direction of where people are going to be. When your wakeboarders are out on the water, it is important to have your speakers at the correct angle to give them the best experience when they are out there. Some speakers will even come with swivel clamps so that you can adjust them as you need to.
Some things to think about when placing your speakers is the chance of people bumping into them, and optimal sound. You want your speakers to be evenly spaced. You do not want them to be too far apart or too close together. Either could give a sub-standard sound when compared to a perfect even spacing. You also want to position them where people will not bump into them. Do not position them in a place where people will be walking back and forth, and there is a chance of someone bumping their head or kicking them with an elbow.
Most towers will have holes where you can mount the speakers. This is generally the best position to mount your speakers. Once you have secured your speakers, you can also secure the wires if you need to.