This span of water separates Berk from Meathead Islands. It is normally calm, but be warned! As the saying goes — when it rains it pours and when it pours be careful of the Shockjaws!
This ocean is described to be relatively peaceful, but once in a while, the weather dishes out storms. While titled as an ocean, it is in actuality a marine area, or sea, between land masses.
In the How to Train Your Dragon book series, this was between Berk and the Meathead Islands. This watery area contains many sea stacks, some large enough to be considered small channel islands, and have vegetation such as grasses and the occasional tree.