Flotsam and Jetsam are Boss Enemies from Kingdom Hearts that appear in Atlantica. They fight as a pair, aiding Ursula in her first form.
Strategy
Kingdom Hearts
Flotsam and Jetsam swim around Ursula's lair, attacking by biting periodically. Sora must focus on casting spells onto Ursula's cauldron. The main spells to use are Fire, Blizzard and Thunder - the best of them is Fire, as it consumes little MP and it is the first on the Magic menu. Flotsam and Jetsam revive periodically after being knocked out, so be sure to keep them at bay. Allow your party members, preferably Ariel and Goofy to take care of them while you focus on Ursula. Or, if you cannot, take them out yourself, and be sure that Ursula is far away from them when you knock them out - this will buy you time to bash Ursula left, right and proper when they are revived.
Kingdom Hearts II
In Ursula's Revenge Floatsam and Jetsam briefly appear but are both destroyed by Magic from Sora's Keyblade.