With those speakers, you will not need a powerful amp and you probably wouldn't notice that much difference in sound quality.
Although your set-up will work and is cost effective, you may want to consider installing a cheap IPod ready stereo instead. Most will be able to power those speakers enough and you would have the "luxury" of adjusting the volume via a knob. I always have trouble adjusting the volume on Ipods, nevermind thinking about doing this in a moving boat. I can just see myself trying to turn it down, end up with full volume and blowing out the speakers.
I recently installed a fusion stereo that the face plate flips down and you just insert the ipod and have great control over it. There were a bunch of refurbished ones on ebay, I got one for 80 bucks. Here's one for a bit more money just for example:
http://cgi.ebay.com/BRAND-NEW-FUSION...item3a5bbadfb3
Perhaps you can find a cheaper refurb or something similar.
Good luck!
rkc