OP, I have listed some small, off the beaten path suggestions below, but your post said "very soon" so when are you planning to travel? Keep in mind that waters in the southern Atlantic/northern Caribbean can get chilly in the winter months. That includes the Bahamas, Turks and Caicos, and Cayman - and probably Cuba too, but I have never been there. I am just saying that you should check out the average monthly water temps before selecting a location if you are looking for warm water.
Dominica is a small island with beautiful scenery, nice locals, and lovely diving. No sharks but we saw humpback whales while we were there - from the boat. But when diving we saw lots of unusual fish like frogfish, seahorses, and flying gunards. We stayed at the Castle Comfort Lodge. It is a resort but a very small, diver dedicated place and they served many local foods. We didn't much care for the area around Roseau, but we haven't been there recently.
Dominica is a bit difficult to get to, we had to overnight in Puerto Rico both coming and going; but since you are going for 2 weeks you could plan to spend a little time checking out that island. Here's the link:
Castle Comfort Dive Lodge | Diving Dominica with Castle Comfort Lodge & Dive Dominica
Another thought, the island of San Salvador in the Bahamas is a great place to see sharks and good diving. I don't know if I could spend 2 weeks there - it's really small - but you could break the trip and enjoy some time on a different Bahamian island. It is a very tiny and quiet place. I remember going on an "island tour" and the driver proudly showed us the "straw market" and it consisted of one little old lady sitting under a tree weaving a bag! The Riding Rock Inn is a small, no frills, resort with family style food; here's the link:
Bahamas dive resort for scuba diving walls, big game fishing San Salvador island
Saba is another "off the beaten path" location with very good diving - but not a lot of sharks - but a very interesting culture. It is a Dutch island and all the houses are white and green with red roofs and have flowers in the garden. It is very steep and rocky but with a rain forest on the top of the island. There is no good area for a cemetery and so all the locals have mausoleums in their back yards. An interesting place.
The Turks and Caicos are a great place to see lots of sharks, but I have only dived that area from liveaboards. You might check out the less developed islands of Grand Turk and Salt Caye to see if you find any place of interest.
Cayman Brac is a small, quiet place with great Caribbean diving and the locals are very friendly and laid-back. There are only 2 dive ops on the island, Brac Shack and Reef Divers. We usually stay at the Cayman Brac Reef Resort - a smallish, diver-dedicated resort and Reef Divers is the on-site dive op. They offer valet diving and your Dad might really appreciate that - we do! ScubaBoard is hosting a trip there in January.
But you could also rent a place and go diving with either op. We usually see some sharks in the Caymans but not many, but lots of turtles, rays, eels, tarpon, and groupers. The island has bird sanctuaries, wetlands, caves, hiking, biking, climbing, and kayaking.
The only places that I have seen mantas have been Kona Hawaii and in Jamaica, much to my surprise!
We didn't see any mantas in the Sea of Cortez but lots of sea lions and we had hopes of seeing whale sharks but that didn't happen. It's a bit colder and rougher than the Caribbean, not so many colorful reefs but nice diving.
Hope you have a great trip!
