I know what you mean about some of the shops showing up. Some would attend (not exhibit) just to see who else was exhibiting and what they were showing. kinda funny.
However, I think such a show could be very similar to a "Hamfest" with a variety of exhibitors. You can even sell different "grades" of booths if the show is big enough (eventuallY) You can have a mix of equipment manufacturer's, dive shops, destination promoters (trips), liveaboards, and even a 'swap meet' section for 'flea market' vendors.
If done far enough in advance, it would be well promoted through local dive shops, the internet (websites and email lists), and dive clubs (and their websites, newsletters and email).
Here's the "burning question". is this "just a show" or do you allow retail sales at the show? If you allow retail sales, how does that affect dealer agreements, sales/dealer territories, etc. ? (I'm sure that won't be easily solved). Since I've never been to a "scuba show" (since there are none in the area) I don't know how they do it at other areas. (anyone know?) However, if you don't allow retails sales, it knock down on exhibitors for sure, which will knock down on paying attendee's.
Some folks will come just to get a good deal on a "show special".
just something to think about...