Before using anything too drastic try flushing the Jabsco with fresh water when leaving the boat for any length of time. Sea/marina water holds plenty of organisms to fester away in the pan and pipes. I normally fill the pan with fresh water, add some "bog standard" cleaning fluid, apply pan brush then flush clear with inlet shut.
Other possibility is waste or stale water in the sink drain pipes. Fit the plugs and see if this stops the smell.