What boat are we dealing with here ? If its the one in your avatar, forget about good sound when your running across the lake (to much noise from wind and headers). The next thing is speaker placement, if you get somewhere where there not aimed at your feet they will sound better and louder. I personally wouldnt waist the time and money with 8"s, if you going to add some bass to it go with 10"s (up under the bow) and a decent class D mono block amp, I'm sure Sony has an amp to do this if you want to keep it all Sony. The reason for the mono block is because it will have more power, generate less heat, and be more reliable then bridging the 4 ch for bass.
My recommendation to start would be to try running the highs (6.5"s and 6x9s) off the deck (mount the as high and as exposed as possible). Then use a pair of 10's under the bow with an amp for bass (even if you use the one you have). I wouldnt use an amp for the highs unless you really need it, it will be more drain on the batt and you will get less play time.
Another option is to run the 6.5"s and 6x9"s off the 4 ch amps front channels and bridge the rear channels to a pair of 8 ohm woofers (or a good single 10", 4 ohm) up under the bow. This will also keep your current drain fairly low (good play time), and should sound good.
