D&Dtools1

You gain the ability to discern the presence of ships around you.
You perceive ships located within a distance of 1 mile per caster level, regardless of current visibility (or even whether or not they are beyond the horizon).
You need not be seeking them to become aware that they are there—this spell will alert you to the presence of a ship if you are below decks or asleep.
Exactly what can be ascertained depends on whether the ships in question are within sight or not yet visible.
Not Visible: You sense only the presence of ships other than your own in the area.
A DC 15 Profession (sailor) check allows you to determine the number, direction, and range to each new ship you sense.
If you sense a ship's direction and range, you continue to be aware of this information until the other ship is no longer in range of the spell.
Within Sight: If a ship is actually within sight, you can gain additional information by studying the vessel for 1 round.
You need not be able to see the vessel clearly—a spot on the horizon is sufficient.
A Profession (sailor) check gives you information about the ship or ships:
This spell cannot penetrate illusions.
Focus: A small disk of ground glass, hung on a golden chain and worn about the neck while the spell is in effect.
This focus is worth 50 gp.