Haven't seen any at sea yet.
Have to take that back, sort of. I've seen 'em over the Atlantic (from the air) and the Pacific oceans, and over the Bering, Chukchi, and Red seas, but from
terra firma, not
from the sea. I have a feeling that the sight is quite different somehow when you're out there on the sea yourself.
Even saw one over Siberia one summer evening, from high in the air, but was mainly absorbed with thankfulness that I was over US or international waters at the time, not
over there. That was an especially lovely sunset, and I have to admit that where my butt was, geographically, added an element to its beauty.