They apparently mark the transitions between several areas or levels of ocean, e.g. shallow to deep, but also the different levels of shallow ocean.
These new borders appeared in a new game as well as an old save in which I discovered new chunks, older chunks already generated were not affected and displayed no such borders.
I suppose these are not intended, thus I file this bug report.
Then again, watching some Nilaus, I spotted those very same borders in one of his videos, so maybe that is an intended change, not just happening to me? Nilaus' video with timestamp when aforementioned borders appear
Personally, I don't like them, but maybe I'll get used to them.
