Stereo Basics.

Most likely will be buying a new 13 Tige R-20 or RZR. Wife can't decide so I want to switch to stereo stuff for now.
Comes with:
Clarion head unit
4 Wetsound XS-650 in boats

Will be adding a set of REV8's to the tower and a XS-10FA sub so sticking with wet sounds so it matches.

My question is amps and how to run them. I am not a stereo guru. What is the difference with 2 ohms or 4. Do I need 3 amps? 1 boat, 1 sub, 1 tower or can I combine? I know I need amps for the sub and the tower speakers. I also want to add one to the in boats. I don't need a pro system, just want it to sound nice and clear and be able to hear it clearly with the boat running at speed. More concerned hearing while I pull the kids than from the end of the rope riding. From what i read these components should do it, just a matter of how to power it. The wetsounds amps are crazy expensive. Is there other options? Also is this something I can do or should i install the speakers and take it in for the amps and wiring? Is there also a tuning process for the system and amps?