I went to Devil's Den a couple of weeks ago, on a Thursday. There were a bunch of snorkelers there, which really isn't an issue once you get into the water. The primary issue was trying to get up and down the ladder to the water. When they had an employee there, it was easy, because they give divers priority, but most of the time I was there, no employee. Expect to spend a few minutes waiting at the top and bottom of the stairs for room to get into the water and back out again. Many of the snorkelers are hesitant to enter the water, since they usually don't have wetsuits, and the water is pretty cold.
Definitely, absolutely, bring a dive light. It's really dark in there, especially if it's overcast. Without a light, you won't be able to see any of the cool details around the cavern walls.
It was an interesting dive, and I'm glad I went, but I don't see myself going back unless I'm tagging along with a friend getting their OW cert.