Great Falls Race for Spectators
The race course is determined on the day of the race depending on the water level, but most of the time the level a few days before will be a good indication already as severe changes are rare in summer. Check the National Weather Service gauge for the current and forecasted level, then check out the event page for the race to see what line is most likely to be taken.
The best place to view the race down the Center and Maryland lines is the observation deck on the Maryland side, a 10 minute walk from the parking lot of Great Falls Tavern Visitor Center. Be there early, especially if you plan on setting up a camera, as space is very limited. PLEASE DO NOT CLIMB OVER THE RAILING, this is a protected area and the park rangers will be all over you (and us).
For the Virginia line, the northern most observations decks of Great Falls Park, Virginia offers slightly more room and a view of the full race course, including a close-up of the spectatular last drop, the Spout.
For the center line, to get the best shot for the last drop down the Fingers, an alternative to the MD observation deck is climbing down to Fisherman's Eddy on the Virgina side and scrambling a bit upstream, as the drop is mostly covered from view for the MD side. However, not much else of the center line is visible from here.
Boaters are advised to get the best vantage point from the water. Either put in at Sandy Beach and attain/portage up a bit, or put in at Fisherman's Eddy and ferry across. Either way, you'll end up with the best seats in the house.
A final not for photographers: It is not uncommon at all to be quite hazy at the Falls on summer mornings until around 10am or so, be prepared.