As I level cannons, I put points in the primary skill, basically allowing me to shoot faster.
I then max out Rapid Reload, as it speeds things up that much more.. alot more when it comes to lightning. Especially since hitting in the same spot usually gives better damage.
I keep one point on everything else to have it, until maxed.
Lightning is probably most popular for maxing at that point, and then Fury.
Explosive is slow, but can be fun to play with as a master pirate, make sure you put at least one point here.
Since we can now retrain, its easier to experiment, than the old days where once a points set, its stuck.
Good Luck matey!