I use magnetic eels which explode when they're near metal rather than on impact. This means that you can set the depth of the eel to about a meter lower than the ships hull. When they do explode it's normally right under the hull and it maximises damage. That said, it can still take 3 to take down a large merchant.
As for the deck gun - were you aiming for below the waterline? If not that would explain why you had to use so many shots. Always aim for below the waterline to sink 'em quicker.
One more thing - don't give up you'll get there in the end - just ask Danlisa how long it took me to work out how to put up my sig properly (something which you seemed to work out straight away) We all get there in the end
