Diving in Tobermory was an awesome experience -- great visibility and cool shipwrecks to explore. We did the Caroline Rose & the Grotto dives with Diver's Den. They met all expectations -- friendly/competent staff, quality equipment, lots of schedule options. Compared to normal "vacation-diving," diving in Tobermory is a bit more DIY -- as in collecting your equipment, bringing it back & forth between the boat and shop, etc.
Also, if you're a newer diver, be aware that no one from Diver's Den actually does the dives with you. This was the first time that we dove without a guide. Lynn had explained all of this to us when I called to make the reservation, so it certainly wasn't a surprise. The person on the boat was clearly an experienced diver, so she gave us site briefings and made sure we were comfortable before we entered the water. Caroline Rose was a super easy dive since as soon as you descend, you can see the shipwreck. With the amazing visibility, the boat & line are always within view. The Grotto required a bit of underwater navigation to find the cave entrance and rock formations. Again, she gave us excellent instructions, but still a new experience for us to be on our own in a new site.
I highly recommend diving in Tobermory and doing it with Diver's Den.