A lot of the diseases of marine origin ( I mean true marine origin and not just human waste being dumped into the bay) are usually cases of a microorganisn finding its way in the wrong host and encountering a hostile situation and it fights back. Because our immune system and natural flora are only well equipped to deal with the usual threats from land and fresh water pathogens a marine one like Vibrio vulnificus can easily overwhelm our defenses once an infection is established.

Bacteria are more common. Viruses are very host and tissue specific and because we arent around marine mammals all the time it would difficult for a virus to cross over between us if one even could even transmit at all.

Cholera is caused by certain strains of a bacterium that attaches itself to the shells of saltwater/freshwater shellfish! At one point it mutated and gained the ability to latch itself onto our stomach lining in the same way, and voila! Cholera.