Question 261283
First you have to buy or rent a boat and get it to a  stream of appropriate size. Do not try to put your yacht in a creek.

Then you have to hire a crew to navigate the stream. And don't forget lunch. WE will be feeding at least 6. I invited some friends over to watch this.

rt=d
we know the time and the distance but not the rate.
r=d/t
24/3=8 mph
36/3=12 mph
12+8=20 20/2=10
The boat would go 10 mph in still water.
So we have a six hour trip for 6 people. Bring enough food and drink. Would you like to get started on the menu now?

NB: boats go knots per hour not mph