Cannon skills are generally slow and have very restricted range, but are incredibly powerful. It's important to note that there are three different varieties of cannon units: traditional cannon, beam cannon, and explosive sword. Beam cannon skills function exactly like rifle skills - high damage, linear, and occasionally a file AoE. Explosive sword and traditional cannon skills focus more on large damage over sizable AoE, though explosive sword skills tend to sacrifice some damage for much less restrictive range. Also, since the majority of cannon skills have wait time, most cannon skills are prime candidates for wait reduction items.
Notable skills include:
Barrage has both the biggest AoE and one of the highest damage modifiers for a base attack in the game. Simply utilizing this base attack while loaded up on cannon-type items can be shockingly effective.
Flashbang is one of the few ways to add delay to multiple enemies at once, making it a decent supporting tool.
Lahat and Pinpoint Bomb offer good damage per cost, but more importantly, allows the user to quickly move enemy units into their normally limited range.
Napalm Cannon, Neptune Ray, and Dolorous Stroke all offer a cheap means to great damage over nice AoE.
Kilauea is very slow, but hits the entire enemy party for solid damage and reduces their attack by 1, making it the only cannon skill that reduces enemy stats.
Blackbeard's Big Boom is by far the fastest high cost cannon skill, especially for its great damage and AoE, and has an interesting effect of changing which way it moves the targets depending on the form Teach is currently in.
A number of cannon skills, both low and high cost, have friendly fire AoEs. Exercise caution while using them.