Blue Ocean is the third stage in Sea of Tuna.
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 of an enemy that is single target attacker and has no wait time between attacks, like the Kang Roos 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, making it easier to stall. This also works against Area Attackers, such as the Ottas here. If you still lose, better upgrade your efficiency/wallet or grind EoC treasures.
Strategy 2 (for 4-star)
Lineup: (A) Combo(s) (I recommend Worker Wallet Level Up combos), Jiangshi Cat (any level; even at lvl 40 it can only survive two hits thanks to Survival ability), Cameraman Cat, Awakened Bahamut, Valkyrie Cat, and whatever other damage dealing units you want.
Start off with spamming Jiangshis and Cameramans. Without Worker Wallet Level Up combos, you won't be able to spam them continuously, but that is alright; prioritize Jiangshi. Cameraman should be able to kill one of the Kang-roos, giving you enough money to spawn Bahamut. Bahamut will then kill the other enemies, giving you enough money to spawn Valkyrie. Bahamut should kill whatever is in its way and reach the base. Some enemies may spawn while Bahamut is attacking the base, knocking him back, but that is okay since Valkyrie should be there to help stall the enemies long enough for Bahamut to return and destroy the base.
If at the beginning the enemies end up reaching your base before you kill a Kang Roo, don't panic. Just continue the level as they shouldn't be able to destroy your base too quickly.