I've been mostly playing subnautica lately so here's some headcanons of the tf2 mercs playing subnautica
Has serious thalassophobia. Jumps at every little noise. After about 60 days game time he still hasn't differentiated between the different animal noises and gets jumpscared by gasopods and crabsquids alike
No sense of direction, gets lost frequently (same, buddy)
Frequently eats spoiled food
Puts some batteries and water in his time capsule along with a picture of the stupidest looking base you've ever seen
Spends a week of his actual life filling a multipurpose room with markiplier bobbleheads
Becomes a Marguerit Maida stan
Is neighbors with a reaper leviathan in the dunes. Names it Liberty. It eats him at least once.
Red, white, and blue on all his vehicles
Puts a thermoblade, a battery, some water, and some cured fish in his time capsule. A good survival kit.
The leviathans are their friends (same, buddy)
Multiple cuddlefish pets and tons of aquariums and containment units full of flora and fauna
Doesn't understand why people are so afraid of this game, they're having a wonderful time 
Has so many hoverfish because hoverfish are the best fish 
Keep calm kitty poster and gorge plushie in their time capsule. Along with a beautiful picture of the cove tree, where they've established a base camp and parked their cyclops
Plays chicken in the lost river looking for crystalline sulfur
Finds the leviathans more of a nuisance than a threat. Everybody's got a dog that tries to eat your seamoth
Learns the hard way that bioreactors are so much more trouble than they're worth 
On the bright side though he has a greenhouse that's just teeming with plants
He's so proud of it he makes it the picture for his time capsule, along with an extremely generous gift of ion cubes
Has a personal vendetta against spadefish (same, buddy)
Pretty much always gathering resources
Scans pretty much everything
Every time he has to find something very specific (vehicle upgrade console blueprint) he doesn't waste time and resources looking for it he just googles the location (again, same)
He could theoretically stay in this game forever farming plants and fish but he does finish it and leaves behind a time capsule full of water so that you STAY HYDRATED
Builds one of those big creative mode style bases you see on youtube but does it all in survival
It takes a lot of time and a lot of resources and a lot of of salvaged metal but he's proud of what he's built
Fills his rooms with pictures of I-posing crabsnakes and reapers trying to eat his seamoth with him inside
Somehow figured out how to get more stalker teeth than he will ever need 
He leaves ion cubes, water, and batteries in his time capsule
Also a Marguerit Maida stan (I didn't say this before but same)
Also scared of everything in this game, but he does love all the little fish
Has lost at least one vehicle to crush depth or leviathans
Old man doing his best in video game
Medkits and water in the time capsule
Number 1 creature enjoyer but also another Maida enjoyer
Scans everything, hatches all the eggs, also has aquariums and alien containment units galore, lots of land and sea plants everywhere
Specifically builds his base in between biomes to maximize resource gathering efficiency
Not scared of anything, even the lava zone is just an ideal spot for a cyclops with the thermal reactor module. Lives in the late game like he does in real life 
Time capsule comes with some supplies as well as a passionate message about how amazing this world is and how much he appreciated his time here, encouraging whoever finds it to see the adventure through and make it out
Pretends to not be scared, is secretly terrified
Another old man trying his best to beat the video game
Gets attached to his cuddlefish and is very sad to leave it behind at the end (has secretly named it Jeremy because it's like a son to him)
Constantly asking the others for help and almost always has at least one person over his shoulder telling him how to not die
Doesn't tell anyone but leaves a time capsule with food and water, a picture of a beautiful base in a scenic location, and a heartfelt message to his fellow survivors