The biggest pollution problem is when the boat sinks. Some boats have batteries in them with pumps. They leak heavily, and they regularly pump the bilge water out into the ocean. As long as the owner has some way of keeping the battery charged, it just continues to do that. It's a source of pollution, but it's not as bad as when the boat sinks.
The biggest source of pollution in our harbours are the live-aboards that don't have holding tanks. They just plain dump the sewage overboard. They live there for years and years, sitting on an anchor and polluting the water. That's the thing that actually causes the seafood and the shellfish that our first nations would like to harvest to not be harvestable, because these boats are typically in closed harbours, where they can actually sit at anchor without blowing away. Also, the bays don't flush. For instance, it takes about five days for the water to flush in our harbour.