When I was a kid in the U.S. Navy, we washed out whites by dragging them on a line behind the ship while at sea. After about 30 minutes they were spotless and looked great. Then they were wrung out by hand, and rinsed in freshwater in the sink, wrung again, and hung on the lifeline with tie strings that went through the belt loops. Yeah, once in a while you lost a pair of trousers, but it was rare.
Unfortunately, the water in Chesapeake Bay and most of the ICW is too dirty to wash your clothes with, especially if the item is white. But, if you're 2 to 3 miles offshore and headed south, just drag those clothes behind the boat for 30 minutes, then rinse them in the galley sink - no wash machine necessary. When soap and water agitation was needed, we used a 5-gallon plastic bucket, clean salt water and dawn dish detergent. The agitator was an ordinary toilet plunger (plumber's friend) - it worked great. This was followed by a saltwater rinse, then a freshwater rinse in the galley sink.