NC as what I am going to write is so specific.
I also have a phobia of sharks but a bit different to you. I don't mind pools or anything but specifically I am absolutely terrified of pictures of them. In fact I've been very tentatively scrolling in case an unkind mumsnetter posts a picture thinking they are oh so hilarious.
I remember exactly when it started too. I had this CD-ROM encyclopaedia thing as a child (mid to late 90s) and I was sat really close to the screen clicking through all the animals and then suddenly a big great white shark, mouth open, appeared on the screen and it made me jump.
Side note - people say they're not scary but why are they always pictured at that angle with their mouth wide open and teeth out? Can you imagine if every time you saw a picture of a lion it was of them snarling, mouth open? I feel like the PR team for sharks needs a good talking to. Although I suspect the "cool" factor of sharks for a lot of people IS that they're toothy and scary. And don't get me started on the weird black dead eyes. I shudder at the thought.
Anyway from then on every time I went on my encyclopaedia thing I would memorise the point the shark would come up and try and not look or squint or peek through my fingers to rush to the next. The avoidance obviously made it worse. It's a completely textbook example of a phobia developing.
Problem is it hasn't ever gone away. Even Finding Nemo makes my heart race. I can't watch the chasing scene. I sometimes take a peek when Bruce first appears with his big smile but it's embarrassingly really scary. I've never seen the lake scene in Harry Potter 4 (despite being a massive fan) because there's a shark head.
In fact I recall one science lesson being made to watch a shark show and I tried to tell the teacher my issue but he laughed at me. My friend, bless her, sat positioned in front of me so I could stare at the back of her head for 20 minutes pretending to watch.
My son has got a magazine with a bloody Megalodon or whatever in it and I absolutely hate it every time we read it and I have to pretend I'm ok but my heart is racing as we get near the page.
I've really tried to expose myself in the hope I'll get used to it but it's never seems to help.
In the moment (like the magazine) sometimes it starts to feel a little less scary but the next time I come to it, even a day later, I'm again paralysed with fear in that moment and my heart races.
The phobia is completely separate to my logical brain. Im actually really fascinated by sharks and find them really interesting. I'd love to learn about them but obviously can't google them.
Sorry for the essay. It's weirdly nice to read this thread as I've never heard of anyone like me before.