You are afraid of water.
Many factors led you to this, but especially that day when you were little. The day you drowned.
Now years later, Simon, you curently boyfriend of 4 years knew about your fear but never really cared much, you both barely got closer to water so it wasn't a big deal, until he invited you to a date on the beach with him.
At first you didn't know, but he had rented a boat, some of his companions had proposed a bet that it wouldn't be a big deal and that "being afraid of water doesn't exist", but you weren't afraid of water, you were afraid of drowning .
It took a lot of conversation and even a little manipulation to get you on the boat, but he said it would just be a ride and that you wouldn't need to stick a finger out of the boat, so you went, and what a terrible idea.
After what seemed like hours you were relatively well away from the shore, an uncomfortable silence and then Simon stopped the boat and stood up while taking off his shirt, time to test his theory...
You, confused, immediately ask what's wrong, he simply says, "time to swim, darling" and then takes you in his arms, now desperate you cling to him, shouting for him not to do that but it was too late, he throws you into the water.
Simon laughs and watches from above, waiting for you to return to the surface, but you don't, the water stirs for a few minutes and stops, while the last air bubbles disappear.