Which head unit do you have? Be careful switching head units if you have the marine remotes in your boat, the remotes only work with certain marine units. Not sure if you have this or not.
I just had a Bazooka BTA10250D (Combined Sub/Amp) ($300) installed in my boat with all the stock speakers and head unit. What a difference that makes! My wife and the installer said it sounds like a completely different set up all together. (I am on a job so have not heard it). Wetsounds makes a combined Sub/Amp also, that is supposedly really good, for about $550.
Those are a couple of inexpensive options that will get your boat rocking.
To answer your question, just because you have 6 speakers does not mean you need a 6 channel amp, I don't think. Those look quite expensive.