When this spell is cast upon a vessel, the ship is surrounded by a protective bubble of constantly replenishing, breathable air and sinks beneath the waves. For the duration of the spell, the ship can travel beneath the water as easily as it did above. While under the effects of this spell, the ship has magical propulsion that gives it a maximum speed of 120 feet and an acceleration of 30 feet. The ship’s pilot uses the normal sailing skill to control the ship while under this spell’s effects, and can command it to dive and ascend as part of the vessel’s normal movement.
Despite the magical propulsion, the ship still requires its crew to perform its normal functions.
The spell protects the ship and all aboard from the dangers of drowning and pressure, but not from damage from outside obstacles or creatures. The bubble cannot be used offensively and prevents an affected ship from rising into a space that won’t accommodate it (like a sea cave too small to contain it or under another vessel). Creatures can enter or leave the bubble at will, but those outside the sphere cannot be forced inside against their will, the sphere proving substantial enough that such creatures may choose whether to enter or to slip away along its sides. At the end of the spell’s duration the bubble pops, causing a ship still underwater at that time to gain the sinking condition. This spell has no effect if cast upon a ship in water that cannot cover the vessel entirely.
Although largely beneficial, this spell can be cast on a ship whose pilot does not want to submerge her vessel. The ship itself uses its Will saving throw to resist this effect.
Pathfinder Adventure Path #56: Raiders of the Fever Sea © 2012, Paizo Publishing, LLC; Author: Greg A. Vaughan.