1: Do not fight them in water under any circumstances
2: Lure them on to land
3: Use a bola
4: Attempt to Tranquilse with bow and arrow or better
Remember: They can only be tamed with fish meat and they usually kill all the fish in the area so you should probably have some prepared beforehand.
swamp 91 fish meat raw cooked prime
river 17 kibble pachyrinosaurus fish meat prime cooked raw
lake 11 fish meat raw cooked prime kibble normal pachyrinosaurus
redwood 4 kibble normal pachyrinosaurus fish meat prime cooked raw
kill hand weapon dino taming fish meat kibble
By far best dino to tame for deep sea exploration and farming. He's incredibly fast on land, out running taptors, carnos, theriz, etc. His swim speed in water is just as good, with megalodons crumpling to his stun and bite. Great stamina and offense, and you can stay away from base for days with this helpful guy.