We ordered ours (6 speaker R series boat) with just the head unit (no amps) as we want to do our own amp and subwoofer setup this winter. Not that Bennington doesn't offer good amp setups, they just weren't exactly what we want and we have the ability to do our own. Haven't checked ours with a multi-meter but based on the fader control, the front 2 speakers are front channels and the 2 nearest the helm and 2 between the rear loungers are the rear channel. Since the head unit isn't 2 ohm stable, I would hope they wired them in series and would certainly think they did.
I'm wondering if with all of the different boat and stereo configurations they offer, maybe they always do the rears on a 6 speaker system in series, such that there would be no issues (other than the loss of some power to the rears), regardless of head unit power, 4 channel amp, etc. Now if they did a 6 channel amp on a 6 speaker boat, I would certainly think and hope they would wire them each independently.
I'm not saying it is the best way sound output-wise for those that have a 6 speaker boat and 4 channel amp that is in fact 2 ohm stable but with all the possible configurations, continuous changes in equipment, the fact that these boats are mass produced, etc., I can maybe see why the more consistent way could prevent some build errors and issues. This would of course be at the sacrifice of some power for some configurations that could actually handle it, noting that many wouldn't notice the difference anyway. And for the small percentage of us (keeping in mind their total annual volume) who do really want the true high-end system and it to be 'just right', we typically have the means to make it that way ourselves anyway.
Keep in mind, we only have 1 boat, wired 1 way. So I can't confirm how they build the others. Just wondering if the above may be the case...