This one is part of a secret of X2, and I love it!
You need Silk Shot and the X-Buster upgrade.
As you enter the underwater base (Bubble Crab's stage), go forward until the large fish pops out from its base on the left. Let it go on its own and have it open a hatch on the bottom. Then slide down the left wall on that opening and you'll find the secret spot in here. Select Silk Shot from your weapons and start charging it. Before charging is done, you'll lose the charge and eight large health capsules pop around you.
Best place to fill your Sub Tanks ever.