I used to try to run a large stereo.....thinking that my stereo was great, then a friend of mine showed me his Bob Carver lightstar setups. to make good crisp sound that is loud takes a lot of power and a really really really good set of amps, and to top that off, properly built speakers. I think he had $30k in the stereo
Back to RO55 and his system. RO55, you will find that adding an amplifier will tax your existing speakers and make them sound crappy as the distortion will be there. so you upgrade the speakers to find the distortion of your head unit. next thing you will find is that you are running out of battery... prior to throwing components at the boat, take a look at some audio shops that deal in the marine market. nothing worse than listening to distortion. as you get into driving power, you may also want to look into at least passive crossovers.
The Pyle amp you shown will be a disappointment. They are inexpensive for a reason. I have a small bluetooth Pyle unit on my tiki bar near my pool - it works ok enough for the waterproof speakers I have however the sound quality only becomes better after the 10th margarita. There is way too much distortion to enjoy it any more than background music.
back to the battery issue. nothing kills a battery faster than an amp. there is a reason most people who install large stereos have multiple house batteries on their boat, as well as a really large alternator to charge everything back up.