These heaters are definately very weak for how they are used. Two to three years on the heater core is fairly typical.
While the heater is basically the same setup as used in your car, there are two major differences that make a big difference.
1) On a car, the water/antifreeze is recirculated. Fresh water can be very corroisve to copper, but once the water has done its damage it becomes "neutralized" and the corrosion stops. On a boat, you are constanly pumping through water from the lake, and you can't neutralize the lake.
2) On your car, you have a heater control. When you don't want the heat, you turn the control to "cold" and it shuts off the flow of water to the heater.
I don't have any recommendations for the problem of the basic corrosion, but it does seem feasible to put some sort of valve in the line to the heater so you can shut it in August. I would expect that you would at least double the life of the heater core if you weren't circulating water through it even when it is 110 in the shade....