My personal opinion...only place one point in explosive. Unless you're in a close combat, it does not go very far and takes forever to recharge. If you do happen to land a shot it does do some nice damage. I don't even have a point in grape or chain shot as I do just fine with thunder or round shot taking out sails and players.
Passive cannon skills are also a matter of opinion. Some of my characters utilize them and others do not.
Rapid reload: does work, but do you need it when you can allocate those points into other skills for more damage? Six one way half dozen the other way perhaps?
Barrage: I cannot confirm if this does much of anything (can anyone?)
Shrapnel: I cannot confirm this either (can anyone?)
I personally would place as much into shoot (firing rate is crucial), round shot (it's free and does nice damage at higher level), firebrand, thunderbolt and fury.
Robert Darkskull
Pistols N Bladez