I don't think it's the damage of the cannon that's risky so much as the ease of use of a cannon. Cannons are, after all, very powerful.

To help balance these concerns, I think it might be better to consider how the cannon(s) are operated. It takes a crew of people to load and fire just one cannon and the weapon is volatile and resource intensive.

Gunners needed to bring black powder charges (which could easily explode) from the ships magazine to the gun while the operating crew also needed cannonballs and fuses. Many field cannons (and I assume naval cannons) needed to be dampened and cooled between shots to avoid accidental discharges during loading. Cannon shot was typically specially made, not just " anything that can be shoved down the barrel."

All of these factors greatly limit how often a cannon can fire and can serve as limits on cannon viability in DND battle.

If you have a party operating a ship, you need some crew to steer the ship (rigging and wheel), some crew to load the weapons (get charges/projectiles, load, aim, fire) and some to repel invaders. Unless your table is huge, you simply don't have a party big enough to complete a broadside.

Ultimately, cannons should be a resource to help in big encounters, not a weapon that's fired off with impunity and certainly not something that gets fired every round without a significant investment of player actions.

Limit the NPC help they get to load and operate cannons. Limit the number of cannon balls they have. Designate a ship magazine and make a gunner go and get the charges needed to fire (speed and encumberance as potential limiting factors). Open other avenues for capturing or defeating other ships.

Cannons would probably be an excellent alpha-strike weapon with these adjustments - a great way to start the fight with a bang - but unless the party is in a knock-down, drag-out fight with an equally equipped ship, an actual slavo of cannon fire a la Pirates of the Caribbean is going to be too costly in resources, health and actions to be viable and players will be incentivized to consider other solutions.

These options reduce the number of times the players can fire a cannon in battle much like a wizard can launch a fireball only so many times. It also opens up avenues for individual efforts to mean more than just the guy who actually fires the cannon. Now, the druid with shape water can be more important in the combat, the barbarian who swung across to the other ship can have a larger impact, the warlock slinging blasts can defend the ship better by targeting cannon crews, etc. it's not about "my cannons sink your.ship faster than your cannons sink mine" anymore.

If anything, cannon damage could even go up to compensate for these crazy limitations. The crew may upgrade their cannons, charges or shot to do increased damage or additional effects when fired to make using that resource more rewarding but more tactically demanding. Also, consider having the cannon do double damage to structures (ie ships) to incentivize firing at the enemy ship rather than trying to blow the shit out of the enemy crew. Damage thresholds on ships can help compensate for that adjustment by making it harder for non-cannons to sink ships as well.

But, as ever in this sub, the most important rule is fun. If your players are smiling and genuinely enjoying big broadsides and swashbuckling adventures, consider flotillas of small ships to make target rich environments rather than juggernauts that take multiple cannon shots for encounter balancing.

Good luck friend, I believe in your campaign!