Of Plymouth’s land area, 44% is located within the Buzzards Bay watershed. The Agawam River begins in Plymouth, and the Wankinco River snakes along Plymouth's town line with Carver.

Carver sits on Plymouth’s western edge, where the Wankinco River snakes along the town line. At its southernmost extreme, Plymouth’s border nearly reaches the shores of Buttermilk Bay in Bourne.