I'm an insulin dependant diabetic (on an insulin pump) and I'm an instructor and tech diver. When I'm diving, post-diving highs are somewhat good. I like my blood sugars after a dive to be around 130-150. I usually start out a dive with BS's around 200. For really long dives, I carry a tube of cake frosting in my pocket. It does take a little practice to be able to use it underwater.
There are a lot of other threads on this board about the procedures some people do before and after a dive. Try a search for "diabetes". I like to check my blood sugars three times before a dive. Usually 1 and a half hours before, an hour before, and 30 minutes before. That way I get an idea if my BS's are moving up or down. I can then adjust accordingly. Being on a pump has made my diving more safe. When I dive, I take the pump off. Since it's off, there is no insulin in my body to cause a low.