What they're good for: I'd say they're good for defense mainly. You could also take them out hunting for fish food.
Where to find: ocean water
How to tame: get a crossbow and tranq arrows. Find a shark close to shore. Shoot shark and swim to shore, swim back out into water and shoot. Repeat this until shark is down. Feed meat and narcotics if necessary. *guaranteed to not get bitten this way*
I personally tame sharks, crypod them and go back to my base where I live right next to a small lake. I put 3-10 sharks in the lake and put them on wander and aggressive mode. If anyone or anything goes into the lake, they'll be dead in seconds. Also if I happen to be getting raided while online, I pick the raiders up with my bird and drop them into the shark infested lake. :)
Megalodon prefer raw meat than raw fish meat, so don't try to kill water dinos to tame your megalodon. In low level, they have low weight, but if you increase its weight, it will be perfect. Megalodon always spawn near red obelisk, so go to the ocean and you will find a lot of megalodon. Remember: If you don't tranquilizater darts, use a crossbow and shoot tranq!