When a girl gives you a hand job using both hands. Fist the right up the shaft then the left....repeat. Kind of like milking a cow only there is only one teat.
My girlfriend asked me if I wanted a regular or riverboat style hand job.
When a couple decide to take their relationship further, with both parties shitting in a bathtub, and then one partner receives anal sex in the reverse cowgirl position. Like riding a boat.
Hey Babe, wanna take a trip on the Egyptian Riverboat?
Much like the motorboat, except you take a runny shit on the woman's chest first. You then proceed to motorboat her, and every few seconds you lift your head up and yell "Mark Twain!"
Shari wanted me to give her a Cleveland Steamer, but I gave her an extra treat when when I gave her a Mississippi Riverboat instead!