So I'm playing mission 7 where you have to blow up the two U-boats, but I don't have enough charges left with the sapper. I think I get 4 in total, and I used one to blow up the bunker in the encampment, two for each cannon and then I'm left with only one to blow up both of the submarines. So, is there any solution or am I screwed and need to restart the entire mission?

Bummer! Had to do the whole mission again.

For anyone else reading this you don't require explosives for the cannons, just shoot the officers manning them. Your man has to be standing in just the right place to fire though, it can be tricky!