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.
The #1 thing that needs to be in TLC 3. At least make it as large and powerful as the mosa. Should have one of the highest attack dammages because scientists have proven that the megalodon has the most powerful bite of any living thing and is strong enough to crush a tractor trailer.