-massive amounts of goo. Electric goo, to be precise. One touch and you'll get electrocuted, causing you to lose a life.
The massive trail of goo leads you to an angry pianta (or palm-tree people) couple who're bickering with one who seems to be the hotel manager. You approach this hotel manager, trying to figure out what's going on. "A massive manta came and covered my hotel in goo! Now my hotel is gone!" press the A button a few more times on his dialogue and...
The paper-thin beast of the sea returns! And with more goo than ever! As Manta approaches, you have no choice but to start shooting water at it. This causes him to slow down at first, then break apart into to Mantas that move a bit faster. As the boss battle progresses, you must continue to spray water at the Mantas, who continue to divide into smaller and smaller segments that move faster with each divide.
All this while avoiding the electric goo in the environment, and keeping your water tank full throughout the fight. Tall locations in the level such as palm trees prove to be useful vantage points for evaluating the state of the battle, while small huts serve as safe zones for the player to rest and think over their plans (this boss is made out of light, impeding them from entering shadow covered areas like the hut.
Once all the mantas have been divided into their smallest form, they will glow read and begin moving towards you, chanting a rhythmic chirp as they approach you.
After you've sprayed down that final manta, guess what?! You've won the battle! The goo evaporates from the level and Hotel Delfino rises once more. A eccstatic hotel manager dances for joy and the level's star is all yours.
As a child, I remember this felt like one of the most daunting boss battles I ever experienced. You have to keep a lot of aspects into account ( water level, life) while being spatially aware of the places that you're stepping or will be stepping in a few seconds. And that's exactly why it continues to be one of my favorites. It's exciting, daunting, and thrilling till this day.