Does anyone know how I could catch a cruise between Jamaica and Belize. I know we could fly but we would have to travel back to Miami or similar where we came from and would like to join a cruise ship for this leg if possible. Can you catch any cruise ships for just a leg of the journey?

It's highly unlikely you'd be able to travel between (just) two Ports of Call as most cruise are not set up to take on or let off (new) passengers at intermediate stops. If you were able to find a cruise line willing to do that you'd probably have to pay the "full cruise" fare even though you'd only be on for one leg. Of course another hurdle you might face, again assuming you found a cruise line with an itinerary that stops at both places, is that the two Ports of Call you are interested in may not be in the sequence you want meaning you could be on the ship while it makes other stops.

To add to what RoamsAround said: It is not as easy as one would think to travel within the Caribbean. As you noticed, many inter-island trips require you to go through Miami. This was the case before Covid. The protocols are all different now with quarantines, insurance requirements, etc.

You'll have to find a cruise that includes both stops and call and ask them. I once had approval to get off early but I would have started at the cruise beginning. And there was no discount.

I suspect you won't be able to get on mid cruise unless it is listed as a starting point. I don't know if Jamaica ever is, so they many not be set up with that kind of staffing.

If you don't already know about Cruise Critic, ask there. Or find a cruise specialist and ask them.