AIUI Deep Blue is a feature-film length production edited from the material used to make the BBC series "Blue Planet".
This was originally shot for SD post production - and used film above the water and SD video below the water. The film was originally telecined to 576/50i SD 16:9 video for post - and the edited series aired in 576/50i 16:9 in the UK.
Two "HD" versions of Blue Planet have aired elsewhere - one was a straight 576i to 1080i convert of the SD edits for KBS in Korea (the upconversion looked pretty good - people were genuinely surprised how good the SD film transfers upconverted), another was a re-edit with the film material re-telecined in HD, and the SD video upconverted to HD. This latter version was the one Discovery aired in HD I believe.
If this is the same movie as the Deep Blue created from the BBC material, then I don't know how much below the water SD video footage is included, and how good it looks converted from SD video. This may not be top knotch reference picture quality stuff - but the photography and content should be good. (Though ISTR that the narration of Deep Blue was a bit "lite" compared to The Blue Planet)
This side of the pond we're crossing our fingers for a BluRay or HD-DVD release of the series "Planet Earth" - which was shot in HD and has aired in HD on BBC HD. It is truly breathtaking - with some amazing photography, and at times the picture quality of the material is fantastic - though it is still the content that makes it.