The Dink Network

as i've levelled up, casting the thunderstrike magic is costing me more and more life. it's gotten to the point that i can only cast it about 6 times and them i'm out of life. the boat repair kits used to cover the cost of a cast and a little more, and now they are not even sufficient to cover a cast. why is that? my magic stat is 23, is that too high? is there any stat i should improve to have it not hurt me so much? my defense is only 14, but that goes down to 10 on the water anyway.
any help is appreciated!