When I flew the C 130 for Her Majesty, our minimum height over the sea was 250'. That was set on the rad alt, and a warning light came on if the reading was below that. Three crew facing forward. The captain flew it, with the co-pilot and engineer's eyes glued to the rad alt. Any deviation below 250' would elicit an immediate "height" call. What this crew did was outside limits and dangerous, but if you are going to do it, set the rad alt at say 150' (which should clear any fishing boats) and have the PM glued to that instrument. I don't think they used it at all; nor the DME.