if there's a place that can be truly be safe from herobrine - then it's definitely underwater. he can't spawn or do his shenanigans there.
and yet, what if herobrine could do something outside it while player is underwater, looting shipwrecks and ruins?
suggestion: add a random chance of herobrine creating an ice platform when players are underwater, (as if he had frost walker boots) so there was a real chance of players drowning because they weren't very careful and vigilant.
This is just a visualizer
Uncompressed audio: https://www.youtube.com/watch?v=ON03hLEfBBA&ab_channel=SpringlessMusic