This level is completely set underwater. Next to you, on the left, there is a chest containing crystals. Another is located in the next room. Maybe eliminate the fish swimming nearby. Go to the next room, where a hammerhead fish swims. Here too there is a chest containing crystals in one corner. Kill the crab guarding the exit and enter the tunnel.
Here you will simply have to follow the path avoiding electric shocks. You will also find another chest containing crystals. At the bottom, before taking the exit tunnel, get past the last electric shocks on the left to get an extra life. Then go back, kill the crab in front of the tunnel entrance and enter it to meet the boss.
The boss will behave similarly to other aquatic enemies you have already encountered. It will always remain approximately in the center of the room and will try to hit you by shooting you with its trident. Then keep swimming (if you get hit, hurry to retrieve some crystals from the bottom). After the boss misses you, it will start gesturing and scratching its head. This is the right time to attack it. Quickly approach the boss and hit it with the tail. The greatest difficulty of this fight is the fact that you are underwater and that your movements will be slower and less easy. However, the time frame in which you can attack the boss will also be longer. Repeat the operation two more times to defeat it.