This portal to Neró was formed due to the intersection of several of Oerth's leylines, the natural invisible "rivers" of mana that form from especially large fluxes in The Weave. Due to the extremely high level of undirected mana in the immediate area of the portal, the sea around it is near-permanently covered in mist and extremely choppy, as well as being full of huge rocks due to underground tectonics.
The portal itself is easily big enough to sail a ship through, though can be hard to see due to the mist naturally surrounding the Western Isles until one is very close.