Smart played in 22 games as a freshman, tallying 30 points.
Marcus Dockery, Aquan Smart and James Graham have emerged as valuable options off the bench in recent games.
“To be on the road, away from our families, not having anyone at the games — all we have is each other,” guard Marcus Dockery said.
Watson averaged 36 minutes per game last season, posting 13.0 points, 3.6 rebounds and 3.4 assists.
Aiken committed to Seton Hall, leaving the Terps with two open scholarships.
Smart is Maryland's second 2020 commit, joining three-star Marcus Dockery.