I think the best questions would be scenario questions. For example:
1. You are on a boat in Molokini crater. The dive guide is describing the dive, which involves visiting garden eels at 130 feet. You are diving an aluminum 80. Is this dive a good idea or not? Defend your answer with specifics (decompression, gas, etc.)
2. You are diving off a friend's boat at a site which is current-sensitive. The boat is anchored. There is a moderate breeze blowing. Describe the factors you should take into consideration for planning this dive. (Location and identifying characteristics of the lie of the anchor; direction to work the dive to ensure an easy return to the boat if you have to surface early or can't find the anchor; tag lines; manned or unattended boat)
3. You are meeting up at a local dive site with a visiting SB diver. You have never dived together before, and the diver hasn't ever dived the site. Go through the dive planning, briefing, gear and buddy check you would do before diving. (Site info, buddy separation protocols, communications, checking weight systems, etc.)
I think questions that make the student integrate the information are much better than a single question that says, "What's the rock bottom for this dive?" It's too easy to knee-jerk out those answers.
(BTW, the things in parentheses are the answers you're looking for, not cues in the question)
Always on the money!!!
1. Narcosis - what is it, why do you get it, and when? (hoping you don't get "happens deeper than 100 ft . . .")
TSandM hit my questions, but here is a variant:
2. You have an insta-buddy, and his/her equipment is different than yours. What do you need to know about his/her equipment, and why? (donation hose, BC dump, gas mixture, weight dump, etc)
3. Describe a proper weight check, and the purpose thereof. (looking for neutral at 500 psi)