35w rmsx2 enough for boat exterior?

I recently bought a set of speakers and an amplifier for my boat. The two speakers going in the interior are Polk DB651 and the two exterior ones are Polk/MOMO MMC650 (6.5"). All four speakers will be powered by a Kicker ZX200.4 amp. Now my concern is with the amplifier; it only puts out 35w RMSx4 (@ 4 OHMs) per channel (the MOMOs are rated at 80w per chan). So for the two speakers on the exterior of the boat that's only 70 watts RMS total. Although my instincts said otherwise, the salesman at the store I bought everything from said that would be plenty of power for an open-air boat environment (he also said he has 19 years of mobile audio experience). I was skeptical, but I decided to buy the amp anyway because it was on sale. My question is, will 70w RMS really be enough to be heard over the loud engine roar (it's a 7.4L beast), or was the salesman full of crap? In my car, I'm sending twice the power to my Polk DB6500s (70w per chan) and that's enough to drown out all ambient noise when cranked up with the windows down. But considering that the engine is three times bigger in the boat I'm assuming it'd be three times louder and I'd need almost times the power (of 35w per chan) to hear the music over it's engine, am I right?

I forgot to mention that the speakers will be mounted in the stock locations (which is close to the ground, in the rear of the boat, near the engine). Also, the exterior space is somewhat small; all passengers will be within ten feet of the exterior speakers.

If you have a thru prop exaust stern drive maybe. Thru hull exaust? Then the salesman is indeed full of crap.There is no such thing as free installation!

I'm not sure what kind of exhaust it has (it's my dad's boat and I haven't heard how loud the engine is). Now maybe I won't have to return the amp just yet... The amp says it puts out 50w RMS @2OHM, so is there a way to wire just the two 4OHM exterior speakers to it to get 2OHMS? My guess is that the amp would see a 2OHM load if the two speakers are somehow wired to all four channels. If so, how exactly would I go about wiring it like that? Thanks.

Interesting question. I don't know how loud your boat is. Remember this though -- it takes double the power to raise the decibel level 3db (this isn't a lot). So, in my opinion either you should go with the 35x4 amp (35watts is actually a lot of power if it is clean power). OR, you should get a much larger amplifier. Going from a 35wpc amp to a 50wpc amp is going to get you jack and squat.New Project: 2003 Pathfinder

Thanks for the insightful response, kfr01. I'm doing the installation today, so I'll keep you guys posted on how everything goes!