I have a Walmart marine starting battery (around $90 last time I purchased) in my Ski boat, We typically get about 200 hrs on the clock a season. Every time we board a skier we cut the engine so it gets a lot of starting. The engine is carburatated 408, at 9.6 compression so its not an easy motor to crank. We have a moderate stereo and depth finder in the boat as well.
In the off season I keep 4 batteries, One starting for the ski boat, One starting and two deep cycle for the fishing boat in the basement off the floor and top off the charge every two months. I set a calendar reminder and I can get it done in a day switching between batteries every few hours. I get 5 years or so from the starting batteries. Thats $20 a year for a starting battery, not bad.
If I was on a big lake or the ocean I would definitely have a second isolated starting battery in a boat.
