Assuming a clear day, you can actually see another ship breach the horizon about 12 miles away. Sneaking up on a ship tends be rather tricky!

Adverse weather such as actual fog (as opposed to "fog of war") or obstructions such as islands will obviously affect this. But generally you can see a long way in either direction at sea during the day, which is probably why they didn't bother with sight range. At night you will obviously be limited by the ambient brightness of the moon and stars or darkvision (which only tends to be out to 60').

Assuming a clear day, you can actually see another ship breach the horizon about 12 miles away (19.3 kilometres or 10.4 nautical miles). Sneaking up on a ship tends be rather tricky!

Adverse weather such as actual fog (as opposed to "fog of war") or obstructions such as islands will obviously affect this. But generally you can see a long way in either direction at sea during the day, which is probably why they didn't bother with sight range. At night you will obviously be limited by the ambient brightness of the moon and stars or darkvision (which only tends to be out to 60').