I think I have seen over 100 hydras in art before. Many of them "scary". This is the first time I've ever had to look away, and actually had a guttural fear reaction to any kind of art. Truly terrifying, David. A new favorite, and new favorite artist.
This is fantastic and suggestive. If I may attempt a critique - not a technical issue but atmosphere/composition - I do not completely buy the rocky cliffs on the background. They convey a sense of "safety", in some way. Such an encounter will be way much scarier and desperate if it took place in completely open waters, without any hope for any eventual survivor to ever reach mainland.