Technically any head unit has an amp inside it.
This is the Alpine plug and play amp that Jordan was referring to:
I've read positive reviews on it and it's an interesting concept. It supposedly gives you a true 45 watts RMS per channel (@ 4 ohms) which is more than double what a typical head unit puts out on its own. Supposedly you can cut the wires and wire it up to any head unit if you know what you're doing but it will plug directly into current alpine models.
For wakeboarding/surfing tower speakers I think you're going to be disappointed with just 90 watts unless you're doing like Jordan says and just for hanging out with the engine off. I'm not a loud music junkie but I'm not really impressed with the 200 watts I'm pushing to my tower speakers. They're fine but nothing impressive.