Hey D&C. I work out of Phuket, and if you were coming here, I'd be able to help more, but FWIW, here's a little information (since your post has been up several days and nobody down Krabi-way has yet offered any advice):
1) You cannot reach the Similans from Krabi (you can from Phuket, but Krabi is simply too far)
2) You might like a 2d/1n trip that goes over to Phi Phi (the best diving easily reached from Krabi) in which you stay overnight on Phi Phi Don. If you don't like that option, you can still go to Phi Phi for diving on a regular daytrip.
3) The top dive spot you can get to from Krabi is the Hin Daeng/Hin Muang pair of pinnacles. It takes quite a while to get there by speedboat, and you usually only get two dives, and it costs significantly more than a regular daytrip from Krabi, but maybe you would find the expense worthwhile if you're an avid diver.
Feel free to PM me.