When I looked out, the boat was already pretty far out to sea.
Literal
Look-across [if/when-と], already boat [topic-は] considerably offshore-[attributive-の] direction [at-に] was.
見渡す ('to look across, to survey, to scan the view') is a compound verb 見る + 渡る ('cross, extend'). 沖 ('offshore, open sea') contrasts with 岸 ('shore'). ~の方 ('in the direction of, the X side') marks a direction. ~と here is the discovery conditional ('when I did X, I found Y').