I have read several posts and articles about how to change the oil on your boat and there are suggestions ranging from pumping out through the dipstick to installing fancy drain plugs and associated plumbing. There are also those who just drain into the bilge and then clean up the mess.
Here is a simple no mess solution that should work for most boats:
1. Get a couple of heavy duty garbage bags and put one inside the other. 30 gallon seems to work the best for me.
2. Buy or borrow sawdust. my neighbor is a cabinet maker so I just raid his garbage can. You can get it at pet stores, home improvement, farm and ranch, lots of places.
3. Tuck the garbage sacks under the engine so that the oil can drain into the opened bags. Pour the sawdust into the bags, the more the better but keep in mind that you will need to remove the whole package later.
4. Open the drain and let the oil drain into the bag with the sawdust, you should be able to pull the oil filter and let it drain too.
5. The sawdust should soak up the oil and make it less likely to slosh around. Replace the drain plug and the filter and then carefully pull the garbage sack out from under the engine being carefull not to catch the plastic and tear it. dispose of the bag and oil in an appropriate manner.
6. I have done this several times on three different boats and never spilled a drop.