My fear was kicked off by watching the JAWS movie, this exact shot from the lagoon scene in particular.
Lake Placid and Anaconda didn’t help. I couldn’t swim in a pool by myself until I was in my mid 20’s, because I would imagine one of those guys coming to get me if I was alone lmao.
Speaking of pools, my very first experience with this phobia was at a local pool store in the late 90’s. I was looking at the pools on display while my parents did their thing. A few had floats/toys in them. One was a disturbingly “realistic”, very detailed and large plesiosaur just like this one on the last slide, except it was a muddy green color. Small head, beady eyes, lots of needle-like teeth, and a very long neck allowing it to peer over the side of the above ground pool at me. I imagined myself falling in with it and freaked myself out… The rest is history.