The Tubeworm Siren is inside the Sea Vent Lab, after you rescue Zapple and get Bonker Tortoise.
The fight starts with a whack-a-mole phase with dirt protrusions. Tentacles will come out and shoot at you if not hit. While it's typically only one tentacle, sometimes multiple can come out.
When the boss burrows, a yellow pustule is on the ground, alternating between firing a slow laser and quickly trying to dodge you. When the laser is off, stomp in the air with
, popping the boss out to be damaged. Spam attacks, use magic if you have it, and she'll be dead before long.