a week ago
MAX is waterproof down to 5m (16ft) without a housing; just ensure that the camera door is closed prior to exposing MAX to liquid.
Check out How Deep Can You Dive With GoPro Cameras? for reference.
Yes. After every use in saltwater, rinse the camera with fresh water.
This is to flush out any remaining debris, salt or sand
Check out How to Clean Your GoPro Camera for reference.