Blue Ocean is the third stage in Sea of Tuna.
Attacking the Base
After the base takes a few hits, the enemy's castle will spawn 5 Otta-smack-u's and will continue to spawn them at a faster rate.
Begin the battle, save money for Bahamut Cat, Kang Roo and Otta-smack-u will instantly die after the attack. After that, spawn non-stop Wall Cat, Sexy Legs Cat, Dragon Cat, Valkyrie Cat and Bahamut Cat.
If you are having trouble with this level here's also a technique with the regular Cat Cannon that can be done here, called enemy syncing. For groups an enemy that is single target and has no wait time between attacks, like the otter stack in this level, the cat cannon's flinch will reset them all to attack at the same time if they are in the same position, making them all hit the same meatshield.