IMHO, there is no chance that you would be "unable to dive".
On boat dives, the DM will either provide you with an Insta-Buddy or simply let you dive solo. If you are an experienced diver, bring along a bailout bottle and they might take the hint & let you do your own thing.
For shore diving, you will not be allowed to dive solo. However, at the popular shore dive locations (Turtle Farm, Eden Rock, Sunset House, etc.), you can almost always pick up a buddy on the spot. I suggest:
- get to the shore dive shop not later than 2 PM (it gets a lot harder after about 3 PM),
- tell the shop guy you are looking for a buddy,
- set up your gear in advance and hang around looking eager to get wet,
- when someone is just about ready to go, ask if you can join them.
Works for me. And - if you get truly desperate for a buddy, PM me.