We are fairly new divers like you and we just came back from Bonaire a few weeks ago. Seriously, you have NOTHING to worry about.
For our OW certification dives, I was considering Bonaire, but, like you, I couldn't imagine going off on our own on a shore dive. I chose Roatan instead, and that was a bit of a mistake. Great place and all, lovely dives (although the weather was bad) but so much of the diving was deep (we were limited to 40' at that time, because of our son.) I wish I had chosen Bonaire instead. Although, Bonaire can get deep, no question, but you can definitely stay above 60' if that is your preference. They don't have walls like Cayman, but gently sloping reefs. Visibility is so good, though, that you can find yourself at 80' and not even realize it!
In any case, it is a fabulous location and you will absolutely not reget it. Buy yourself one of the shore dive guides--they are ranked for difficulty of getting in and out. Usually, the sites closer to the ends of the island seem to be the toughest. Otherwise, there are plenty of dives that are easy and wonderful. And you can always do boat dives as well.
Being so close to the equator, the weather is pretty much the same all year, from what I understand. It was hot when we were there, but the breeze makes it quite comfortable. I didn't find it overly crowded (this was for Christmas) but I understand is is near deserted in August. And, from what I hear, you can get some really good deals at that time on airfare and hotel.
#1 reason is: Bonaire is out of the hurricane belt. August is prime hurricane season. I'm originally from Miami, and that means something to me.
There is an internet forum for Bonaire which many locals and old-timers frequent--an excellent source of information. I got all my questions answered--and I asked a LOT of them. I hope it's okay to post it here?
http://www.bonairetalk.com
Good luck with whatever you choose! I've never been to Cayman and it is definitely on the short list!