I've got two landmasses that are each situated on either side of a 20-50 mile wide sea (with the width varying along its length) where the sea runs for several hundred miles before opening out into the ocean proper. Is there any way that the tectonic plates of my planet could move away or toward each other to create this? If possible I would like to avoid too high levels of volcanic activity and earthquakes although having at least some would be preferable. If a formation like this isn't possible what would be the most similar? (i.e. when the Americas moved away from Europe and Africa at what width was there a stable sea between the two?)
In this sea what would the currents and weather systems look like? Would it be a fairly calm and easy crossing or extremely perilous at most times of year?