[Puddin'_Tain]

Pour a cup of bleach down the drain, let it sit five minutes, and then pour a bucket of freshwater down the drain (or stop up the sink and fill it with the house water, then pull the plug) to flush out the bleach. Five minutes of bleach isn't going to do anything appreciable to the pipes. And a cup of bleach isn't going to do anything appreciable to the environment. Also, try to "flush" the sink (either with a bucket of water, or by stopping and filling the sink) every once in a while to prevent a reoccurrence.

 

[Puddin'_Tain]

They are right about the bleach, and about using it sparingly.

If you are concerned about the bleach you can also use hydrogen peroxide to kill smells and it is harmless to most pipes and such, as well as wildlife, etc. You want to use the standard drugstore 30% active type, use generously, and let it sit for a little while. It's not quite as aggressive as bleach, so you may have to repeat.

Trust me, I'm an R&D chemist for a company that makes cleaning supplies and deodorizers...

The "standard drugstore" hydrogen peroxide is 3%, not 30% H2O2 in an aqueous solution. However, you can get ~30% hydrogen peroxide that is marketed as a "cleaning solution" (rather than as a topical antiseptic) at some hardware stores, hydroponics supply outlets, aquarium shops, etc. Strong solutions will burn the skin, and are probably about as dangerous as household bleach. Industrial grade peroxides (including >30% hydrogen peroxide) can be downright nasty to use, and should be avoided unless you have some experience with such chemicals.

I would just used regular bleach; it's quick, easy to get, and cheap. But the ultimate source of the problem is organics floating inside of the sink drain plumbing. A little bit of cooking oil, some food scraps, and whatever else doesn't get flushed out will turn into a stinky brew. Many (most?) boat freshwater systems don't deliver water at a high enough rate to effectively flush that gunk down the hose and out the thru-hull. That's why I suggested pouring a bucket of water down the drain once in a while. You might also try scrubbing the inside of the hose with a long-handled bottlebrush to remove any mold/mildew growing in there.

 

[Puddin'_Tain]

If it's just "A little bit of cooking oil, some food scraps, and whatever else" what's wrong with using a plunger? It's an awfully good test of your hose connections and clamps, but other than that, it should blow anything in the drain, out.

If the plunger will force enough air through the plumbing to displace all the liquid, then it would probably work just fine. The trick is to get everything past the thru-hull. If there isn't an actual clog then the plunger shouldn't really be "pressure-testing" anything (as long as the seacock is open).