There are lots of things that go into getting great blues. A major one being - having great blue water in the first place. I will spend a lot of time on dives looking for that angle that gives me the best blue and then finding a subject to put in front of it.
Another factor, which I don't believe applies to you Fras! Is having good technique. Too many people these days are lazy and don't get close enough to their subjects. As a result they do not get quite enough flash onto their foreground and have to correct it using the WB in the RAW converter. In the processes of reducing the blue in the foreground they also take blue out of the backgrounds.
These days we see a lot of UW photos with grey-blue or purple-blue backgrounds because people are over correcting their WB in the RAW converter.
Alex
