Wild mantas can be fairly hard to hit with a megalodon.Just keep your shark still and the manta will often swim in front of its head.the rest is easy enough.with a high level megalodon even 120 mantas only take a few hits
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.