I thought I'd create a guide good if your starting out to Subnautica.
When you start, the tutorial explains all of the controls, right? Yeah, not so much...
My first tip is that if you're losing oxygen, you need to get up fast. Try L-Shift (Left shift). That sprints.
F11/F12 Allows you to take screenies, but I recommend you use options to switch it to F6.
Lastly, Space bar is jump and that will help get up above the surface for long periods.
Okay, so maybe I'm not the best person to ask about this particular subject: see screenshot.
This is the only way I catch peepers. Luckily, I swore off survival saves after that.
You need a good frame rate for this, but you can also use quick reflexes. Alternatively, get the knife and spam right click. Then, hope you hit a peeper as it flies across your screen. I'm sure it works. Well, good luck catching peepers!
Firstly, water runs out fast. Make this your first priority after mastering the controls, or it could be your doom.
The only way to get water is to catch bladderfish (NOT recommended, but if your doing a try not to rage video, give it a shot), or getting salt deposits broken to make bleach. If you can find it, do it.
Things Never to Do!
Here is a list of things not to do, early game. This are common mistakes, don't make them.
Neglect your search of bladderfish / salt. Water goes down fast, so this could be your death, in fact it was my first. Go behind a gaseopod. I am not kidding, instant death.
Getting the Knife
So, first, you'll need silcone rubber. Then, you need to get 2 titanium, which is pretty easy. Find metal salvage (large bits of metal chillin' in the crash zone area, near your pod usually), which'll give you 4 titanium when put in the fabricator. Pretty sweet, huh? You can also find it by breaking limestone outcrops.