An increasingly popular diving activity is known as "muck diving". It is diving over a muddy, silt bottom and doing what scuba divers do... search and view the unique critters that make that home.
Some of the best muck diving in the world is in Raja Ampat and more specifically in the channel on the south side of Batanta Island.
There are many unique octopus, crabs, worms, bobtail squids, etc. that thrive here.
Equipment: Bring your own if you can afford the surcharge on the airlines due to weight limitations. Otherwise rent it locally.
Raja Ampat, along with having some of the most bio-diverse life of any ocean, is home to more seahorses than I've seen anywhere. There are dozens of species of these little hard-to-find pygmy hippocampuses.
To see these guys, you will need a good set of eyes and lots of patience. They are very small, some of them no taller than 1/4" or 6mm. They can be camouflaged very well and move very slowly and deliberately.
They wrap their tails around the stems of fans wire corals. They can be found elsewhere, but these are your best odds. Best we found while diving was 9 seahorses on one fan!!
Equipment: Bring your own if you can afford the weight penalties on the airplanes, otherwise rent locally.