We dove bat islands last month with rocket frog divers, and I would say we are fairly inexperienced divers (we were certified last year, and had ~20 logged dives before going to costa rica), so I don't know how helpful my opinion on the currents will be. The conditions we experienced diving in costa rica were pretty diverse, but in general, the visibility and currents changed from dive to dive, and even mid dive. We were diving with a group of much more experienced divers, and there were definitely dives where people were holding on or crawling across the bottom, or hiding behind formations, and where the visibility was fairly poor. The day we dove the bat islands the currents weren't that strong at the big scare (the first dive site), and the visibility was fairly good (20+ feet), but there were much stronger currents at black rock, the second dive site. I could definitely see the potential for injury at both sites if a inexperienced diver didn't swim out far enough from the pinacles before ascending.
We didn't see any sharks at the bat islands, which was a bit of a disappointment, but I still thought it was well worth the trip - we saw a fairly large octopus, and tons of larger fish that we weren't seeing at other dive sites, including a a lot of trigger fish. Most dive ops won't take you there without seeing you in the water beforehand, and most also have a minimum amount of people they will take, so even if you decide to go, it may be hard to arrange.
If you decide to go, Rocket Frog Divers the dive op we went with have was excellent, and have one of the best/fastest boats to get to bats. We also met some locals, and they had good things to say about deep blue diving adventures.