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.
These are great for doing fish meat runs. Most water mounts such as Basils, eat fish meat as fast as they can gather it. Others such as spinos simple are a pain to use in underwater combat due to turn radios and hit box. Baryonyx gathers fas much fish meat as a spino, but better due to its stun ability and unlimited oxygen and maneuverability. Just swim right up to a megladon, stun it, chomp. Rinse and reappear. Best at fish meat gathering in my opinion. Never underestimate their stun ability!!
The Baryonyx is a large crocodile-like predator. While it might lack in attack power, its ability to swim extremtly fast makes it a fierce opponent — especially with its ability to travel on both land and in water.