I think if you dont yet have any sea tames, sharks are a good start, and your best bet for getting one is a baryonx or iguanodon. Bary is an obvious choice, but it isn't so easy to find a good one, so on a budget, a high level iguanodon is very underrated and will easily do the job. It is not so fast at sea so upgrade some move speed (you should anyway), but it is a decent fighter, typically with good enough weight and stamina (upgrade health and melee).

OR… a sarco, spino, a thylo, an anky, whatever you have that can swim really, would take out other lower level sharks easily, and then you can tame it with a dolphin.