Deep Ocean Blue is the second stage in Swimming Cats.
Sir Seal can be annoying to kill with its good attack rate, damage, and HP, and so can Croco, because he's just a meatshield for Sir Seal. Celeboodle won't be a huge problem, but she can plow through your lines.
This stage is very easy and can be done with 3 meatsheilds, a cow cat, and Valkyrie cat. start by waiting until the sir seal is almost at your base. Once he is, start meatsheilding to stall. When you have enough money spawn Valkyrie cat. The rest of the level spawn valk when you can, meatshield, and use cow cat as a gap filler.