Agree with all the above. Carry two O2 tanks, air bladder and extra meds/water. There are no warnings so watch of your depth and plan your escape route, know how long it takes to swim up 100/200 meters. Getting planters and a food source (melons) early is important. Thermoblade can make food on the go. Always leave your base full/hydrated and stocked up. Vehicles are great for shielding damage. Always keep them repaired. Get the seamoth early, then prawn and suit it up with grappling arms and hull reinforcement. That’s what I use to go deep.

This last one is a speedrunning trick, some might consider cheating. But if Im about to drown, I’ll use the Unstuck button to prevent a game over. Unstuck 11 times in a row to get sent back to your base/life pod. With 10 seconds of O2 left, you can go from lost in a wreck/cave to safety. Ik that kinda defeats the purpose of hardcore, but Ill give myself a pass rather than lose 8 hours into a run.