Best ice I've found S of C Cod is in Oak Bluffs at Nancy's. 10# blocks of dense ice for $6.
For short cruises, I make my own blocks using the bags from 5L box wine. Just fill with water and lay them in the freezer.
Least expensive ice I ever bought was in Lake Montauk - an ice house on the E side of the inlet, oriented to the fishing crowd. Told them I wanted ice, and they just swung the chute over and filled my cockpit until I yelled 'Stop!' - $3.