It depends. The river system is at 'full pool' from mid-May thru Labor Day, but there is a year round navigable channel; check https://www.tva.gov/Environment/Lake-Levels
for immediate and projected levels. Most marinas are open 7 days during this period. It can get pretty rural upstream of mile 350 or so and some marina restaurants and bars may be open only on weekends even in summer. Most cities and towns try to emphasize their water interface; Chattanooga (M460) has done it particularly well. Water temperatures are warm enough for wet sports and recreation from mid-June through September. Fall colors are good usually late october or early november. The navigable river is 652 miles long. It'll take a day or 2. Come visit.