Since the charge is housed in water it will not destroy your cannon, but the shot is not housed in water when activated, so it will destroy nearby blocks and injure nearby mobs. To properly fire an active shot (and avoid destroying your cannon), you need to be sure that your charge activates before the shot is activated.The charge (if you are just playing or testing) should land (TNT will fall when activated) and explode in water, otherwise the explosion will destroy nearby scenery and damage nearby mobs.The charge is an amount of TNT used to propel the shot.

If raiding, make sure (unless you are using a hybrid TNT cannon) that the shot does not land in water.

The shot is the ammunition it can be a player, a mob, a piece of TNT, arrows, sand/ gravel or anvils.TNT cannons operate on the principle that when TNT explodes in water it will not destroy blocks, but will still launch entities, including TNT that was already ignited, which is usually done with redstone (the charge is usually ignited immediately, then after a delay the shot is released).As of 1.8, it is also possible to build a TNT cannon that works with pistons and slime blocks. Note: these (as the whole article) talk mostly about the classical TNT cannons that work by shooting TNT by using other TNT blocks as charge. 9.6.1 Select fire, select range TNT cannon tutorial.9.3.8 Human (or other entity) launching cannon.9.3.7 One-button defensively ranged cannon.9.3.3 'The Pulverizer' - An Ultra Rapid Automatic Slime Cannon.4 Limitations, and ways to possibly get around them.

Minecraft vertical slime block launcher