That's a tough one. The only place we've run into that has been an upscale marina (though I know some lakes have that policy). You can put a dishpan in the sink and pour it into the toilet. No way to contain the shower; you'd have to use the onshore facilities.
We find this to be a more often concern in campgrounds on land as opposed to on the water. I made a fitting with a hose to go into the sink thru-hull to drain into a 5 gallon bucket or into the campground's sewer hookup. We use the campground shower. Or, the easy option: back to the dishpan and dump it in the sewer hookup.
HTH
Best wishes,
Jim B.