Hi Jaan! Trip was TREMENDOUS! I'd give Poseidon a miss though: Spanish Dancers was the best dive shop in Zanzibar that I found---at least in Nungwi. Didn't check out Memba, but it looked like there wasn't much down there. I checked out Poseidon in Nungwi and they didn't hold a candle to Spanish Dancers. All the dive shops charge about the same, and Spanish Dancers was by far the best outfit I saw. By August things might settle down, but as of now you have to have a PCR or NAAT test before arrival (preferably PCR) and do another rapid test at the airport on arrival. BY ALL MEANS COMPLETE THE VISA APPLICATION IN ADVANCE ONLINE AND PAY FOR IT IN ADVANCE!! My companion was smart... me, not so much. Online visa process looks kinda sketchy but it's legit and saves TONS of time at the airport.
DO NOT TAKE THE FERRY FROM DAR TO STONE TOWN! Huge mistake on our part. Fly: the baggage allowances on Tanzanian airlines are pretty generous.
As far as diving, Zanzibar good. It's not Palau or Belize or even Honduras.... but I saw a lot of things I've always wanted to see: cuttlefish, seahorses, weird skates, and many red damsel fish (the latter, a personal obsession).... Diving was relaxed and uncomplicated. Spanish Dancers prefers that you come the evening before you dive and rig up gear---which I really appreciated. They use metric steel tanks.... 12L and whatever the larger size is. I found the 12L sufficient. DIN or yoke--no worries there.
Beach in Nungwi is very fun. I stayed in a couple places but Aman Beach Bungalows was the best value for the money. Aman is on the beach and Spanish dancers is right next door and you can either walk on the beach or through the various hotels and resorts. Best food is at the fish market right near Aman towards the north. If you want a cool party, walk all the way up the north beach to KangiBangi hotel. Very nice people there.
Beach is pretty safe even at night---only had one run in with an overly drunk and aggressive vendor, but he was quickly dispatched. If you walk way up to the north end at night you may want to carry a flashlight.