I can't speak for Cuba directly, but just in general, on all tourist beaches in South American countries, detectors are a common site on tourist beaches. You could certainly ask some official or consulate there, but then you would get this psychology:
You might try this instead: see if there are any dealers there. Whites, Fisher, Garrett, etc... have lists of all their distributors and dealers. See who is closest to there, and if they sell detectors there. Then simply call that dealer, and ask what the actual situation is. Of course, being a nearly 3rd world-country, Cuba may not have any western-made items to begin with. There are a lot of Cubans in southern FL, so you might check with dealers at that end of the state, to see if they've dealt units to persons who frequent Cuba too.
You can't metal detect in Cuba. You certainly can't go to Cuba with a metal detector on your bag when flying, nor even the checked bag. If you go on a boat with a MD, and if you try to MD, you better do it in hiding, if a police officer finds you MDing, try to act as if you had no idea, the PO might be linient, or you might have to pay your way out, or you might end up in jail if the PO is a die hard Communist.
Anything of value found in Cuba belongs to the government, and noone else. That's the law in Cuba.
Do people not know that Cuba is still a Communist dictatorship?
That claim of Senator Barbara Boxer that "Communism doesn't exist anymore" is a lie, and shows how i g n o r a n t people in power are about the world. There are billions of people still living under Communist dictatorships.