A boatbuilders definition would be somewhat different.
A 'blue water' boat would have inherent/redundant strength at 5-6 times of the 'normal service design'. 'Coastal design' at about 3 times safety factor. 'Inshore design' at twice the inbuilt strength (safety factor).
Inotherwords a design-specific 'blue water' boat would be built at a minimum twice as strong as a 'coastal' design and 3 times as strong as an 'inshore' design.