and worst case scenario you have to start one engine for 15 minutes to charge batteries back up.

Just dont even go for this scenario...you will look like a tool infront of your boating friends.

Whatever makes you and your family happy do it!
Who cares what some douchbag with a boat thinks most are tools themselves.

I wouldn't charge the batteries with the alternator. The batteries or alternator well not like it. A stand alone charger would be a better bet. When you get home charge the batteries. The batteries and alternator will last longer.

Evidently you have never been next to this person in a tie up....they look stupid. Every couple hours have to make sure noone is around the rear of the boat...and then listen to him/her rev the motors enough to get the alternator to put out a decent charge, drowning out whatever good tunes and good times are happening. Its annoying and it ruins the mood of a nice relaxing day.