Easy and endless money/XP/trophy farming (quite cheesy) (AU DLC required; 100% vanilla with no mods)

First, and most unfortunately, you need to get the DLC with the Australian reserve (Emerald Coast). It costs IRL money. I'd suggest waiting for a sale (it's 3,11€ right now). It's not the most enjoyable reserve for me, but it lets you do this stupid cheesing exercise.
https://store.steampowered.com/app/2383430/theHunter_Call_of_the_Wild__Emerald_Coast_Australia/

Next, if you have no money, arm yourself with any weapon, it doesn't really matter, as long as it's not .22LR or something similar.
If you have money to spare, get a hard hitter like .300, .303, .308 or greater. My personal preference is to stick with handguns (since the game has an atrocious inventory management, I hate changing my loadouts every hour or so, so I just pack up all the handguns I can and try to settle for shots at shorter distances), e.g. .454 or .45-70 (this one is paywalled). Ultimately, it doesn't matter, because you're doing money farming, but if you're into trophies, then you need a decent caliber and good shot placement (and no more than two shots per animal; I've unintentionally managed to get two diamond crocodiles while grinding for a few hours). By the way, bow arrows (and probably crossbow bolts) don't work for some reason.

The cheesing is possible because of a basic gaming industry principle: illusion. Since consumer-grade computers are not capable of serious simulations, most of what you see is clever developer trickery. The typical examples are bodies of water and skies. Thus, "projectiles" in this game behave in "water" exactly the same way as they behave in "air" (except for arrows/bolts, as mentioned above).

So, after saltwater crocodiles spot you, they will hiss and either go berserk at you (and take a bite off you), or they will run towards "the water". It doesn't really matter, because once they charge at you and, in case you fail to incapacitate them, they will still hide into the "deep water" and just stay in a fixed position like a literal sitting duck (it's bizarre).

The tricky part is to find a "water" edge that's not shallow enough for you to be able to go prone there. There's a "sweet spot" that you'll get the feeling of once you try it a couple of times. Going prone into "the water" lets you see what's below it. What's left to be done is for you to fill them with lead and harvest. It's easy to spot them, they're usually at the deepest part of "the water", and their eyes are almost glowing (this effect is easy to see during the night). A good (lethal) shot will be given away by a typical sound effect, which you'll learn to recognize after a couple of times (it sounds like something's kicking inside the water, very asynchronous with them being still).

The hot spots for this cheesy exercise are the swamps in the North-Eastern/Eastern parts of the of the reserve. I don't recommend the sandy beaches on the Eastern edge, because it's hard to go prone in the water there.

If I can find the time, I'll make a video soon.

Cheers.

1. Get the Emerald Coast Australia DLC.
2. Go to the swamps in the Northern-Eastern part.
3. Start running like crazy, until you hear hissing.
4. Try going prone on the edge of "the water" and take a look inside it.
5. Pew-pew these dinosaur mofos that are sitting still like the AI dum-dums that they actually are.
6. Profit!
7. Say "thanks".