The Silent Sea is a erratically appearing spot in the Oceans of Lumen Orbis where all natural currents are disrupted by magic Depthwalkers use to move around the ocean. This Magic causes from time to time that currents break apart as parts of them are redirected by the Depthwalker.
This entrails that inside these Areas, the magic of the Depthwalkers does not work as creating entirely new currents is too great of an exhaustion to keep up for long.
The Depthbreakers, known to hunt Depthwalkers can operate within these areas as their muscular finned bodies do not require the aid of currents to move.
Thus, they are know to lure and drive Depthwalkers into these areas to deprive them of most of their magic.
The Water in this areas is only being moved close to the surface by the waves but further down the water is stagnant and unmoved, creatures being able to swim on their own are unaffected but others that ride currents get stuck in these zones and have to wait for the currents to normalize, which can take from a few hours up to even a few days.
The Silent Sea can appear all through the Bottomless Ocean, sensitive creatures can feel the shift of the currents. The affected area is not clearly geometrically defined and usually has loose borders that shift during its existence.
Reminds me of doldrums, that are areas with basically no winds near the equatorial line. But doldrums don't have any magic involved, I suppose.