Is it plausible that a virus could contaminate the entire ocean to where humans that enter it are highly likely to contract the virus? It doesn't necessarily have to be the whole ocean what with the pressure from the deep depths and the temperatures of the N&S poles, but anywhere that a person could swim in.
Virus and bacteria, like any other life form, need a suitable environment to stay active.
Virus in particular need a host to reproduce, while if they are in the outer environment they usually degrade and become inactive.
The time a virus can remain active outside of a host is variable from virus to virus, but in principle it is possible for one to stay dormant for a long enough time.
The main problem I see is that, being humans not exactly a water species, it is unlikely to have a dedicated virus attacking specifically humans in water, since the average time to find a host would be extremely long. It has to be an opportunistic virus attacking water animals/mammals.