These guys are definitely the top 3 best tames for water. Don't waste your time with Megalodons, Kapros aren't nearly as fast (only unique factor is their ability to dismount enemies). I'd say aside from this absolute beast, Baryonyx is a good alternative. They're fast on land, have a combat buff of stunning and blinding (to my knowledge at least) and are good for cave exploration. But for real, Sarcos are the little kings of the water.
I just knocked one out. I was very afraid of these monsters before but its actually a quite easy tame! I recommend this snappy thing for beginners underwater mounts. Good luck taming it. [Up so the noobs see ;D]
DO NOT listen to the person who wrote the following comment: “DO NOT tame in the lower south cave. It will get stuck on the way out and be trapped there forever.”. If it can’t (which will most likely be the case if tamed down there, because (in my experience) Sarcos struggle just entering the cave with how wide they are. But don’t listen to that guy. Just summon it out in the “Request Creature” tab and wait for it to teleport (I have summoned creatures out of the cave when they get stuck and they teleport just fine). Share this with anyone you know who struggles with that.
For beginners, these crocs will make your life on ARK so much easier and you will NOT regret you obtained one. You can ride them at lvl 35. They are super fast swimmers and even travel quickly on land too. To tame quickly:
1 ) Shoot with tranq arrows until unconscious
2 ) Fill it's inventory with Fish Meat or Prime Meat
3 ) Protect your croc from nearby enemies
4 ) Enjoy your new dino !!!!!!!!!!!!
Pro Tip: These guys are great for obtaining tons of meat on land, but bring lots of meat/fish in your crocs inventory before travelling into the water, the croc will eat and drain it's food supply very quickly while underwater. Fish gathering I DO NOT recommend with this dino.