Over four plays (two in theatrical and two in curator mode), I've discovered certain ways to keep people alive. I have yet to play a fifth time and try each of these in the same playthrough. I'll be happy to get feedback. Spoilers are ahead, but I'll try to minimize them.
Julia: Don't surface too quickly after the dive. If you do surface before Alex tells you to, make sure you don't drink the beer. (On Curator Mode, she surfaces quickly and drinks the beer so you cannot save her life.)
Connor: The easiest way to keep him alive is to successfully steal the boat near the beginning.
Brad: The only time I got him killed was when Connor did not steal the boat and Brad failed a QTE chasing Connor.
Fliss: If you don't tell Brad to come out when the water rises, he will end up by himself. He will find a wrench and soon come to a monster that is not attacking him. You will have the option to attack the monster. Don't attack it.
Alex: The only time that I got him killed was when I waited too long to drop the heavy door on Olsen after Danny had been killed in the water.