Very typical of D&D. Just say that these sharks are an Oerth variety, rather than a match of Earth sharks. In fact, many animals would be different on Oerth (changes in habitat, appearance, etc..) due to the differences in environment (different landmasses, environmental regions, etc...). Or, just say that they are bull sharks.
As for the Sahuagins.. Drop the fresh water thing? Or make 'em a subrace of Sahuagin. D&D is FULL of them.
ba-da-bing, ba-da-boom