I have recently been reading up on a game called Polaris (a post apocalyptic underwater role playing game) and am thinking of putting together a version using Genesys for my group, but was wondering if anyone had put any thought into making Genesys a bit more dangerous. Options I can see include: subsequent critical hits can have a higher bonus to the roll, suffering more than twice the characters wound threshold being lethal, also I suppose that falling unconscious while underwater could be quite dangerous anyway.

There are some ideas here: https://community.fantasyflightgames.com/topic/269416-how-deadly-is-combat-in-genesys/

You also might look at Deep Dark Blue by Evil Hat Productions which is a FATE game, but there's a lot of concepts there that might be worth a look.