AFAIK, Blue Juice is a small shop in Krabi Town. You might contact them directly and see whether they do in fact have operations on Koh Jum.
As far as the diving in the area goes, I'd say it's forgettable. I drove over to dive from Koh Libong last spring and would not make the trip again with diving as a priority. The water is silty (5 - 10 m viz only) and there is nothing of particular note about the marine life other than dugongs. I did spend about 90 minutes in nearly zero viz for a shallow (3 m) dive in the seagrass area where the dugongs live looking for them, but saw nothing interesting other than some murex shells crawling on the bottom. The dugongs, if they were there, probably heard me coming and moved out of sight--but I'd have to have pretty much bumped into one in order to see it anyway!
You will most likely not be able to dive Hin Daeng/Hin Muang from these little islands. The dive boats are not meant for open ocean travel such as you would need to do to reach these pinnacles. To dive HD/HM you will need to depart from a more developed dive destination with operators who have bigger boats--from, for example, Koh Lanta, Krabi, or Phi Phi. From Phuket you can get a 2d/2n LOB that goes down there.
Now if what you are looking for is that 1960's-beach-shack-backpacker experience, you should definitely go to Koh Libong or Koh Jum or one of the other nearby islands such as Koh Kradan. Transfers are complicated and do take a long time. It would take you the better part of a day to get from Libong by longtail to the mainland and then by mini van from the pier at Hat Yao to Trang where you could get a bus up to Khao Lak (not Ko Lak--it's not an island but a town on the coast). If you've got that kind of time, go for it! It's well worth visiting some of the less developed islands on Thailand's Andaman coast.
