A family business, United has been offering scuba supplies, rentals and classes to Massachusetts's shivering scuba diehards since 1970. Its Washington Street shop façade is covered in nautical murals and flies a red and white scuba flag. Inside, rows of masks, flippers and wetsuits hang from every vertical service, and glass cases hold mysterious and shiny tank parts. United has literally everything required for a day beneath the (icy) waves.