Outboard Battery Charging

The heater thing can definitely work. Look up shunt type battery charge controllers.

Also, now that you have posted a picture, one of my concerns doesn't really matter. I had figured that the cost of the additional primary wire needed to work off of one battery might well cost more than the second battery - especially if you needed to run separate leads for charging and starting to each motor. That isn't the case with your boat.

One last thing. There have been three or four threads here that discuss the troubles of a single motor on a catamaran. There is a way to make things work okay, if not perfectly. You need to place a wedge on edge in front of the leg. Think of a 4' 2 X 10 bevelled to a point and butted back to the leg. This will stop the motor from drowning itself, but it won't fix the prop from lifting. You should be able to extend the leg of your existing motor to at least a 30" leg. There is no substitute for prop depth on a displacement speed boat. Performance and economy go way up if you operate on open water at modest speeds. I was able to keep a 28' boat's prop in the water with a merc 15 at 30". It had a 5" spacer added to a stock longshaft leg. The power head was about an inch above the waterline, but the prop had bite and the motor lasted 15 years in tropical salt water year round.

 

I'd check to see if your 10hp actually has a regulated charging system, many dont
Regulation on an outboard just means short to ground anyway being a permanent magnet charging system.

 

Re your water diverter to the propeller
While this will push the water down, it will also induce a drag on the boat

Re The AGM battery
You will have to check with the manufacturer about the charging voltage and float voltage of your battery and see if it is compatible with the output of the alternator of the motors. AGM's require different charge and float voltages.

RE a 12 volt heater
Most cigarette lighters in cars run off about a 15 amp fuse. Not a lot of heat for a 15 amp draw. But I guess some heat is better than no heat

 

I concur with powerabout.
Small outboards have just a coil and a rectifier, designed to produce nominal charging current in a depleted battery. At 14 V the charging current approaches zero.

 

With a "boost converter" you can make an excellent regulator. It will deliver output voltage that you set, say 14v, constantly even when the supply voltage drops/fluctuates at idle even. Only the amperage will now rise or fall but the voltage remains constant at 14v. You can also set the max amperage output as well.

http://www.ebay.com/itm/Boost-DC-DC...458?pt=LH_DefaultDomain_0&hash=item4adcd22cd2

 

At six amp capacity, most simple OB's coil and rectifier can't do much harm to a standard battery anyway. But at 2 X 12 Amps into one small battery, and considering the OP's environment, I think a controller of some sort is prudent. Can always bypass it if it acts up.