Re: Cold water. I got my certificate in '98 - this heatwave summer was the first time I ever dived my own country. Before that Cannes (France) was the furthest north I’ve ever been.
I don’t have many dives under my belt, but I have been pretty lucky with the locations:
The best place overall was the Whitsunday Islands and Bait Reef (part of the Great Barrier Reef), Queensland Australia. I saw small rays, mantas and everything else… apart from sharks.
Second best was Sodwana Bay, KwaZulu Natal, South Africa. Really remote place with nothing to do but diving and fishing (and having your shit stolen by monkeys). I stayed for a week. I saw everything (still no sharks) including a flock of dolphins and whales jumping in the horizon. You should go.
I also dragged my 7mm wetsuit through 5 weeks of African heat (actually KLM lost one suitcase, so my first night in Johannesburg I only had wetsuit, knife, flippers and mask - no clothes) because I wanted to do a sharkdive in Cape Town where the water is cold. On the boat the day before my flight home, the divemaster looked at my expensive suit and said:
That’s really a nice suit… unless you want it smelling of fish guts and blood forever, you should probably borrow one of ours…
The dive never happened because the waves where to big and the one Great White we saw only stayed for 2 minutes - I did touch it’s snout, though (5 meters at least).
I took my certificate just outside Sydney Harbour which was surprisingly nice. I was the only student so on my final testdive my instructor took me to some caves (his secret spot) where I saw 15-20 sleeping Port Jackson sharks.
I had a bunch of nice dives in Hurghada, Egypt. Beautiful easy access reef and lots of fish - unfortunately it’s greatly overpopulated and reef conservation hasn’t been a priority before now (now they have a lot of Russian divers, who just doesn’t give a shit and don’t listen to instructors, so their reefs are suffering).
But it was good and cheap.
My personal wishlist is:
Shark safari in southern Africa - The Big Five underwater starting with Tigers and ending with Whale Sharks. I also want to do my Great White cage dive without a storm ruining it and South Africa is the best place in the world to do it (cheaper than California and Australia by far and with an almost guarantee of sharks)
Wreck diving in Egypt - there’s some big WWII ships.
Great Barrier Reef again
Thailand.
Belize/Bahamas.
Local wreck dives. We have WWI and WWII ships. A WWII sub and a jet fighter just outside the beach where I grew up… I must get over my dislike of cold water.