At the age of seven I kept having this recurring nightmare where I would find myself in the middle of the ocean. I remember I felt exhausted, I tried to swim but I could not move from the spot. Things took a turn for the worse when I noticed an enormous shape ascending from the depths, it was heading towards me, really fast. But I really felt horrified when the beast emanated a deep growl that felt like a shock wave. Fortunately for me, the dream ended at this point.
I can't say this an accurate depiction of the sea monster from my dream but it's close enough.