Are these speakers going to run on the head unit's internal amplifier or an external amp? If the HU amplifier, I kind of doubt you'll notice very much difference between them, particularly when underway. Both will likely sound a lot cleaner than the stock speakers (presuming your boat has the same crap speakers as mine!) but the weak signal from the HU will limit volume and bad distortion will rear its head at anything approaching a volume you could really hear underway. Also, I'd be leary of any claims of strong bass down in the sub 50hz range, as that's just not really physically possible. In short, an amplifier, even a fairly small one, will likely make a much bigger difference than whether you go with the Polks or the Infinitys.
FWIW, I had a pair of Polk db 691s in my Dodge 2500. My experience may be the exception, as they sounded fine for a pair of low-priced 6X9s but fell apart after about a year. The crossover piece was just spot soldered on and fell off, resulting in loss of signal to the mid/woofer. I resoldered them once but they fell apart again. Granted, these were in the doors of the truck, so they got slammed around a bit with opening and closing. Regardless, the construction certainly wasn't what I would call robust and I wouldn't expect it to survive particularly long in a boat that sees any degree of chop regularly. I'm not sure if the 6.5s are built more solidly.
. I have six of the 6912ms installed now and hope to add the two in the bow over the winter. 
