Re: Marine Head Unit with 6 Speakers?
Same as in cars. Two ways around it actually.
First, if 2 of the 6 speakers are tweeters, they likely use a crossover on a component set. The crossover magicly marries the tweeter and woofer/mid to give one standard 2 or 4 ohm output for that channel. ( So you have a set of components in the front and regular speakers /coaxials in the rear with matching 2 or 4 ohm impediance.)
Second way is to run 4 ohm speakers in the rear and 2, 2ohm speakers (per side) wired in series in the front. This also gives you 4 ohm's on all 4 channels. (BTW, 4 ohms is pretty much the standard in automotive /marine speakers. There are others, but selection is limited. )
In either case, you really need a decient sized amplifier to run 6 speakers. Headunits, even the good aftermarket flavors, just don't put out enough wattage to make 6 speakers sound good. All the ambiant noise you get in a boat makes the situation even worse.
hth