It seems strange that the charger would not accept a lower amperage as 20-30A at 240 V is far more energy than 15A @ 120V.
I suppose ideal would be to charge both truck and boat from home/overnight stop, charge truck near boat launch while you are out fishing, and then charge up boat as you drive it from one launch to another. Just be sure to account for the efficiency loss transferring energy from one battery to another. There is another thread on here talking about using Lightning to charge a battery electric plane that may be informative for your use case.
I suppose ideal would be to charge both truck and boat from home/overnight stop, charge truck near boat launch while you are out fishing, and then charge up boat as you drive it from one launch to another. Just be sure to account for the efficiency loss transferring energy from one battery to another. There is another thread on here talking about using Lightning to charge a battery electric plane that may be informative for your use case.