There are the divers (AKA, the thieving invaders!) and the protectors: Mutant Shark Warrior, Manta-Man, and Squid Warrior. Click the picture to enlarge it. I like using LEGO figures, because if you have the right gauge of straw you can mount the pieces right on the straws and minimize their contact with the frosting. Fruit rollup pieces (see the "seaweed" under the Squid Warrior?) are also good, because only the very bottom of the LEGOs get sticky, and soap and water will clean that right up.
The gateway and larger seaweed trees are from an Imaginext set.
Christopher's actual birthday was the next day, so I couldn't use the figure from his new Atlantis set on the cake. I bought this set because it was relatively inexpensive, and I liked the warrior. He has atrocious underfangs, and an arm+fin combo that looks kind of like the wingalings on Trogdor. I think of him as a Sea Monster Warrior, but LEGO calls him the Barracuda Warrior. Whatevs-- he's weird, and therefore fun. He doesn't have a magic shield, though, like the other characters. Christopher claims it's because he's too stoopid. \o?