If you have to stay on the mainland, then Placencia followed by Hopkins would be my choice.
There aren't any other viable options. Staying in Belize City for more than a night while you transfer either to or from diving would be a bad idea. There is a dive shop there, Sea Sports, but they cater to the cruise ships.
If you go to Placencia, there's a wide range of options for accommodations. I'd recommend Splash for diving. Thy would also help you find a place to stay. (Disclaimer: I'm very good friends with the owners/operators but regardless I think they're the best dive shop.) If you stay in Hopkins, take a look at Hamanasi.
Or maybe Hank's house in Sittee River, lol. (j/k, Hank)
Thank you Downing and everyone else for the great info. We were able to book the Blackbird Cay Resort
Now I'm counting the days.....
I will report back with pictures and videos..... Here is a small teaser from my trip to Roatan, Honduras in March:
View media item 211100View media item 211099View media item 211098View media item 211097View media item 211096View media item 211095View media item 211094View media item 211093View media item 211092View media item 211091