Yes I know it's painfully uninformative and boring but for the love of god if you don't play it you don't even know a single bit on how to play this game. And this guide can't help you if you don't know how to buy guns. (Flares too, for that matter.)

Yes it's effective and nearly everyone uses it sometimes at least when the sharks are lurking about
BUT
even when your team took out all the sharks. This is a tug of war and if you don't beef up, you go home in a soup can.
Don't camp with backs against windows/doors. A shark can just blitz through the room and ram you out with it. Breakable walls apply here too, if a shark is punching holes in it, stand clear.
Tank sharks include the infamous hammerhead and the Great White with over 90k hitpoints. HH can nail you into walls for good while GW can wallbang you to death as fast as your Wilhelm Scream.
(Mako and Thrasher). hug your juicy corner so they won't come zooming by and sling you miles away for a free kill while your team can't do ♥♥♥♥ about it. It sucks, trust me.
try to stay away from openings but remain mobile, they usually have limited lunges so they'll pick static (camping) dudes first. Don't be that guy.
In short, don't camp the corner like a chicken. Camp when the situation calls for it.
Enuff said.

When you die (you WILL die. A damn lot. They don't give you fat purses for no reason), don't just groan at that bit of lacking cash for big boss ADS and spawn in with you peastol again. If you can afford bigger n better guns, nobody's gonna stop you buying em, but if you still have spare cash left, don't just spawn in.
The ocean floor is dark. Your flashlights are weak. Sharks like darkness. Don't be the simp peeing in his wetsuit. Be the chad lighting up the mess and ruling over sharks. Flares give you light and tag sharks that comes in close. Tagged sharks are more or less harmless.
Healing your teammates gives you 250 points, lets them survive longer in the next engagement for a bigger shooting window and preserving tickets. Plus it's a pain in the sharks' asses.
It's especially useful against those sharks that evolved bleeding attacks. You can tell it from giant amounts of blood loss, Wilhelm Screams, and depleting health. Without medkits wounded divers must venture out into the dark open waters to bandage themselves, or bleed to a pitiful death.
Buoys are the bigger cousin of the flares, with longer lifespan, more response range, an audio alarm, but being more expensive, only applies blinking tags and can be knocked out by the sharks. But they're still a good buy no matter if you're crossing open water to another safe or securing your room's premier. Just don't slam them in a doorway so a shark can silence them without much effort, putting your 100$ to waste.
or sentry, is like a diver with okay aim but doesn't give a ♥♥♥♥ about leading shots, and they're 1hk. Overall not worth your cash.
may prove useful for blinding sharks, especially if they have fast ones that rely on Intel...but not the thrasher. The thrasher can scrap these toys from outside your room, enuff said.
is great for setting up a premier that no shark can sneak pass...if your teammates are decent. Or they can just grab stuff off your crate and...try to incubate them? Or even worse, dropping a dreadful mine in dumb places that blows up teammates, give STEVE concussions, and doesn't deal with the shark at all.
Some love them. Some hate them. I don't place them much. They're grumpy. They explode everything in range. Try to experiment with them if you got one from a supply crate but never NEVER place them where you might pass by.

Now for the good stuff, your only trusty companion and lifeline in hostile, shark infested waters, your guns.
good ol classic cheap and effective starter weapons, able to take down up to mid sized sharks with 4 rounds, good accuracy, okay reloads. Don't try to spamshot with these guns, their magazine is too small to create a lead wall. Only fire at cracks on the walls/doors/windows or a shark that's partially static (usually munching down your team mates).
Sharks will take more than 1 second to kill a diver usually, a big window to react and gun it down. That's why you don't want to stand in the open because sharks can swoop you out where your teammates don't have LOS and chow you down with ease.
Also, don't stick too close to each other. Sharks will still travel after catching a diver mid-lunge, and if that means it stopped behind you, you'll have to turn and orientate, wasting precious time.
If you are an AWP main in CSGO this might be the gun for you. 1 shot, big damage(for T1 guns) means high stakes/rewards. If you can't land shots that accurately, I'd suggest you to stick with standard guns. Or, take
Basically a palm sized Mac-10 that will spit out 12 rounds of shark hating lead down range full auto. Ofc it's less hard hitting than other guns but when you're facing small fast sharks, Lead walls with bleed/spread may be your only salvation. Ammo will be a problem, so you still gotta make your mags count.
6 shot, hi-power, signature shot sounds. Ah, old west but underwater. CUT. Old west is dead. Sharks won't don cowboy hats and meet you high noon for a duel. Sharks will chow you and your sheriff star into tiny pieces and spit on it. Unless you're a deadeye. In that case, at range this gun can unleash ungodly amounts of firepower but with mild recoil that spreads rounds out. Overall not for freshers.

Your portable leadwall generator. "Spits out rounds like there's no tomorrow". If you don't murder these sharks fast enough you don't have a tomorrow anyways.
An ump but underwater. Does what's said on its label. Cheap, full auto, 25 rounds(with ext. mag upgrade), reasonable price for spec. shots. Your mid game go-to gun against fast sharks. These bad boys will tear them apart quickly either with pure kinetic force or by bleeding them out. Bleed rounds are insane for stopping sharks bouncing around the room by making them pay for their speed. Doesn't pack enough punch to kill those bigger sharks though.
A bit outdated Russian AR. Has similar traits with its end-game successor ADS aside from a slower ROF. You have your standard round roster on this gun too and you can be sure that some rounds will land the shark and apply its effect.
The thing that makes this gun unique and worth its price against bigger sharks is the which beefs its barrel to withstand more payload, and that means more punch per shot! It basically turns this gun into an automatic marksman rifle, but reduced ROF makes it struggle against faster sharks.
And for the big boi:
Any diver's wet dream. 30 round, supreme ROF, okay damage, this thing deletes sharks at mid-close ranges. A cheaper 'I win' button than the harpoon gun.
Honestly I've never used this gun but it's just a bigger microjet launcher. If you work out well with the high noon gun then this can be the gun for you.