my experience with boat audio it focuses more on mids and highs than it does actual bass.. because you really cant hear much when your underway..
do some powerful mids and highs backed with a amp.. and you will be fine..
also some 6x9s in moveable boxes isnt a bad idea.. because he can place them on the nose of the boat if he is at a island docked so people around him can hear and he can place them back in the boat when hes underway.
if he wants some bass while docked then go ahead and install a few 8 or 10 inch woofers around the back seat area.. (if you have some interior pics i can throw some ideas out .. but be sure to include a bass knob so he can turn it down while driving..