Not too far away from the Washington Monument and the Lincoln Memorial. I would recommend walking from the Smithsonian Metro.
Photo Spot Details
Needless to say, Tidal Basin during the Cherry Blossom Festival gets very VERY crowded. Best time to photograph would be around 7 AM during a weekday, when DC residents are working and tourists are asleep. Tripods are allowed but actual use may be challenging if you go during a peak hour just because of the sheer number of foot traffic.
I wouldn’t recommend expecting to take a great sunset photo. Tidal basin area is not exactly a great location to get beautiful sunsets.