Depending on supplemental charging from solar cannot be relied on. The continuous draw of 250 watts - about 20 amps - will require a battery bank larger than 1500 AH. 2000 AH is closer but even that may not be enough. I hope the boat is large enough to carry over 1200 lbs of batteries. Not to mention the expense. A generator is a much better solution, combined with a charger.
Living aboard in Victoria Harbour