That sounds like a really good idea. Is there some major drawback to having such a thing? There must be, because otherwise it seems like they should become common or even required safety equipment.I would think that, for most dive boats, a prop cage would be the most sensible idea.
You know divers are going to be in the water so if there is any chance that a diver might be in the vicinity of the prop you should be planning for it. On most rigid dive boats in my experience the ladder/lift is at the rear therefore near the props. If you can't guarantee the engines will always be off with divers down, you need to ensure that the props can't cause damage.