Make yourself a longbow with plenty of tranqarrows, then build a big enough cage going from shore into the water with trapdoor and ladder close to the Dino gate, build a small platform to shoot from. Get it to chase you and wait for it to come thru the gate, then close the gates to trap it and fire on it till it K.O
Getting one megladon makes getting more a lot easier. For what ever reason it aggros wild ones quite far. Just swim underwater and spy class the one you want. Lead it to the coast and begin tanqing. As for underwater usage. My 225 6k health and 364 melee was able to take an alpha megladon solo. I did manage to kill a lvl 70 alpha but needes help from rexes. Fairly good underwater would recommend 2 or more for higher level alphas though.
Use a high level Basilosaurus with high health to lead it to shallow water where the basilo gains health faster. Dismount and let the megalodon bite at your basilo while you traq it. Tamed 4 150 megalodon with this method works like a charm.
Be cautious while taming. Your megalodon WILL attract other megalodons once tamed, just like ichys. Make a saddle while taming so you can head to base ASAP, or exterminate any nearby wild megalodons. If you kill other nearby sharks, you'll get some prime fish meat to help speed the taming process!
You can’t eat or drink underwater which makes healing or renewing your Fria Curry or Lazarus Chowder a problem - Megalodons can swim straight up and straight down unlike some other aquatic mounts and when surfaced, you can dismount and stand on them to eat or use healing tonics. With enough HP you can set them to passive and let them tank damage from a Megalodon while using your rifle to dart the wild megalodon for a faster KO. Always best to have a few sharks with you while exploring the oceans.
South Haven is a great spot to look for these to tame, especially in the bay in the middle of the island, all you have to do to tame one there is kill all the fish in there besides the megalodon you want (I suggest using a baronyx to kill the other fish with its stun ability) and then tranquilizer the one you want with a crossbow.
1. Reinforce your raft with stone foundations, they won't be able to break them.
2. Make them attack your raft and shoot at them with tranq arrows (Bow and crossbow can both be used but a crossbow will be quicker) when they swim away because they are almost unconscious make sure you hit all the arrows you shoot so they won't escape.
3. Tame them with the food of your choice (high level Megalodons will need some narcotics or bio toxin, you won't need it immediately but be prepared).
4. Have fun with your Megalodon!