Actual disease spread is not some magical contaminant that will instantly infect you if even one germ gets onto you.
Transmission of disease occurs when you come into contact with sufficient bacteria that it manages to multiply and breed inside your body.
Depending on the bacteria or virus, it may take hundreds or thousands of bacteria to come into contact with a person to infect them.
Bacteria in an infected person's fluids are likely to range in the hundreds of thousands of germs, so direct contact with even a drop of someone infected may cause you to be infected.
However, if you put that infected person into the ocean, the bacteria will spread out, and given there are millions of gallons of water in the ocean, the amount of bacteria will likely be maybe 1 or 2 per every few gallons of water, and you'd have to come into contact with thousands and thousands of gallons of water to get infected.
Not only that, but since sea water has high salt content, most bacteria would simply die off. So sinking a ship is not likely to be a problem, unless someone swims right by the bodies.
(Covid19 information below, skip it if you're not interested)
THIS INFORMATION IS RELEVANT IN REAL LIFE AS WELL.
Covid19 requires you to come into contact with a certain amount of infectious load to become infected. If someone infected is outside in the open air, their viral spread will be dissipated into the open air by wind and be unlikely to affect anyone, UNLESS THEY'RE RIGHT NEXT TO ANOTHER PERSON. You can also decrease the amount of virus you receive if the infected person wear a mask properly, reducing how much virus they are putting into the air, and if you wear a mask properly, reducing how much virus you breathe in.
THIS WORKS FAR LESS WELL IF YOU ARE INSIDE WITH AN INFECTED PERSON. Even if you are far away from an infected person, if you are in the same room as them for long periods of a time - 4-5 hours and there isn't any ventilation like open windows or air conditioners that actually circulate the air outside, the virus they are spreading will slowly fill the room, and give you a far higher viral load. And the more people who are in one area? The more likely SOMEONE is going to be infected in that group. Masks help slow the time, but it will still be a higher risk in an enclosed space.
WEAR A MASK. KEEP YOUR DISTANCE FROM PEOPLE. STAY AWAY FROM CROWDED AREAS. TRY NOT TO STAY IN AN ENCLOSED SPACE WITH STRANGERS. WORK FROM HOME IF YOU CAN. It's not always possible to do all these things, but the more you can do, the less viral load you will likely get, and the less chance you'll be infected badly, and you may not even get infected at all.
It's not all or nothing. EVERY LITTLE HELPS. Do what you can.