the size of the boat is fine, IMO it's more the motor type you have to worry about it. i would not run a prop....you'll smoke it. run a jet, you're ok. lower in the river you'd probably be ok, but you definitely have to be careful. a smaller boat with a little motor you'd be better off, but your 20' boat with a jet would be the ticket. both are big rivers and see 20' boats all the time.