Well, no matter what type of stuff you use,,if the servo is hard locked and sitting there lockstalled, it's gonna put a huge strain on the system until you click it off,,in which case,,the genie is usually out, or has his bags packed for the next available rug.
It's all about matching components, settings input, and monitoring the expected life of your gear as you go. No one is immune from not having there tried and true method(s) cooking servos. That's where the expendable income can play a huge factor, in how hard you run your equipment, to a lesser degree.
To error on the side of safety and plenty of capacity for a light boat, 1100mah 6.6v LiFe,,for your answer. It's about 65 grams and it will last a very long time. For your Mono 20 boat with those servos, you could get away with 800mah with light loading..but I would keep a closer eye on it. About 43g with 800mah. Captain Obvious says you get to decide on weight and capacity my friend.
More variable and constantly changing load on the steer servo in a boat, than in a pan car, with all things being equal or close to that.