You can mount the amp in the engine compartment just be carefull to find the coolest place. There is lots of air movement so that is actually not a bad place. DO NOT mound it upside down. You can run two subs off one channel but one off one and the other off the other is prefered in boats for temperature reasons. I would go with the single 10 over dual 8's. Also get a 4 channel amp and a 2 channel or a 5 channel amp and run all your speakers off amps. The head unit will not be able to keep up with amplified power at all, especially if you are going to have seperates.