The pilot reported that, during maneuvering for external load operations to a boat, the helicopter suddenly began to shake violently. He immediately realized the helicopter had struck the boat with the main rotor blades; he jettisoned the 50 ft longline, the skid-based emergency flotation system was activated, and the helicopter landed on the water without further incident. A post-accident examination revealed that the helicopter had struck the boat's mast and sustained substantial damage to the two main rotor blades. The pilot reported that there were no preaccident mechanical failures or malfunctions with the helicopter that would have precluded normal operation. The pilot further added, he should have measured the boat mast height prior to operation instead of visually estimating the height and instead used a 100 ft long-line.