DeCaster (2-0) fanned nine and allowed one walk and five hits. The senior improved his three-year Tommie record to 12-8 with a 2.46 ERA with 179 strikeouts and just 40 walks.
Andrew Tri pitched a scoreless ninth inning and struck out two.
Ryan Lust chipped in with two hits and also scored two runs for the Tommies (4-1).
St. Thomas made the most of its seven hits and capitalized on five walks, four Whitewater errors, and two hit-by-pitch. The Toms scored three in the third inning and a pair of runs in the fifth.
The teams play another nine-inning game Friday evening here in Pensacola.
Whitewater has been shutout just twice by Division III opponents over the last three seasons, both times against the Tommies.