24 volts from a 12 volt supply

Bad idea Dave.

Beside the awkward wiring around the engine and the house system's minus being elevated 12V above ground, one of the batteries will be charged to death.
Series charging is quite normal (the 6 cells within one battery), but the state of charge must be equal. If the house battery has been heavily used and the starter battery is almost fully charged, the 28 volts from the alternator will be divided into 12 + 16 V, slowly killing the starter battery.

Nothing beats the solution in post #3.

 

The easiest way is to mount 2 - 12 volt batteries, close to the thruster, go to my earlier post and buy the charger from Minn Kota, You run 2 wires, +and-,about 10 gauge, from your engine battery to the charger. The alternator on your engine will charge the 2 batteries. I have used this setup for 2 years with no problems.

 

I know it is an old post but maybe for archive purposes, I charged my batterypack (24v, bowthruster) with two small (1' x 1') solar panels, switched to 24v. the panels were spec'd at 10w, but as the bowthruster is not needed on a daily basis usually, they always kept my batteries fully charged