OK, I suppose I should weigh in with my guess...
It seems pretty likely that things will start to open up around June 1st, however I suspect that diving will not restart immediately. Mexico will likely take the phased approach to reopening that the USA is trying, and will have about one month of watching the USA's results. Delta resumes Saturday service to CZM in June, although SWA isn't scheduled to restart until August.
There will be a few logistics to work out; many workers from elsewhere in Mexico went back home when everything shut down and would need to return. But QRoo already has an ad campaign planned,
Tourism Council announces reopening of Cancun, Riviera Maya travel for June - Riviera Maya News, and everyone seems to be pinning their hopes on the estimates that the virus will be gone by the end of June. I suspect we'll see the navy direct the port captains to reopen the ports by mid-June, so I'll put my $.02 on 20-June for resumption of diving. And by that I mean that you will be able to book starting June 1st, but for a trip no earlier than June 20th.