Seems like Knox County auto tags are about $34-$36. Boat registration is dependant on length and the number of years you want to buy. I think up to 26' it is $20 year, up to 30 something feet it is a little more ($28?). If you buy more than one year you save $1 for each year you buy. I've never registered for more than one year, but I never seem to keep anything for more than a year either.
What kind of boat do you have? I usaually don't boat up where you're at, but sometimes we take a friends cruiser up to downtown. I'm at Fox Road Marina and usually hang out between Prater Flats and the dam.