Williams was 8 of 10 from the free-throw line. Eric Fanning added 11 points for the Seahawks (5-4). Williams and Mario Moody had seven rebounds apiece.
The Mustangs led 30-26 at halftime, but they only made 18 of 53 (34 percent) from the field for the game. The Seahawks made 18 of 37 (48.6 percent).
Nick Russell led SMU (8-4) with 20 points, including three 3-pointers. Russell played all 40 minutes for the first time in his career. Jalen Jones had 14 points and 14 rebounds. Ryan Manuel had 15 points.
Wagner will make its second consecutive appearance in the Cable Car Classic title game, playing the winner of the Santa Clara-Alcorn State game Saturday. SMU will play the loser.