How to tame a meg:

One have a baryonyx or sarco to swim down there

Two have a cross bow and get close to it with tranqs make sure to have meat

Three wait for your rex of the sea to tame

Four bring it back to your base

Five craft a saddle

Six ride your rex of the sea into battle

Seven feed it fish meat when tamed

Eight build a wall/little pen around it

Nine if you don't build a pen keep a eye on your meg!