Barotrauma

• Any injuries under aprox. 20% are not worth wasting medical supplies on, with the exception of bleeding ("minor bleeding" is fine to ignore), poisons, and husk infection which you should always try to clear asap. Anything less than 20% should be treated by fruit juice (pomegrenade extract, you can set up a botany room and grow the fruit on the sub) or via lying in a bed (veeeerry slowly heals injuries over time). At outposts and cities you can also spend a fairly minor amount of money to top people's health off at the medical doctor vendor.

• Supplies wise you should be keeping an eye out for minerals and plants mid-mission. I'm not familiar with all the vanilla subs but some should come equipped with a mineral scanner, and you should absolutely be stopping and sending people out to harvest them with plasma cutters. Very very early you might tread water a bit but you really shouldn't be spending most of your money on basic things like welding fuel etc.

• This is more an RP choice vs minmax choice, but you can also pretty easily rob blind any outpost or city you visit, whether it be of medical supplies, clothing to turn into organic fiber, or diving suits to turn into titanium/aluminium alloy and rubber. As long as there is no NPC in a room you're looting, you can freely yoink whatever you want. Two people working together makes it easier, as one person can grab and walk NPCs out of a room if they're standing up, wait till the door is closed and then loot goblin it up. This is a thing you'll have to decide with your crew though.

• Conversely once you make it to the second biome (assuming y'all are playing the campaign), you'll start finding/getting missions for wrecks which you absolutely should strip relatively clean. Or if you end up fighting pirates/enemy subs that's even better loot. Just uh, try not to approach wrecks from directly or diagonally above. Or do, I'm not your captain.

• Guns, once you get access to things that aren't just SMGs and revolvers, are far and away the better choice if you have the resources to support it. The harpoon gun is great because you can re-use the ammo of course which can be great for long away trips. There will be things later that require you to have better weaponry when you do out of sub-ventures.

• If you still feel like you're struggling, in the campaign options turn the amount of missions you can take up from 1 to 2, or even 3. They may not always be in the same direction but being able to take on multiple can really help get your economy going. It also makes it more exciting! Multiple swarm kill missions, or a swarm + big monster? Gotta find and retrieve an alien artifact that causes fires while transporting a VIP to the next station over? What could go wrong?