I believe this is a sea snake based on its distinctly textured skin (snake eels are smoother); the flatness of its head; the inability to see a dorsal fin (although those can be flattened out); and my general sense of it while observing. Too bad I didn't photo the end of its tail, which would help confirm snake vs snake eel.

I wonder if it's unusual pattern is due to its living in a jellyfish lake, which is a habitat where a salt water lagoon/lake is enclosed completely inside an island but maintains a connection to the sea.

About the lake:
https://en.wikipedia.org/wiki/Kakaban