Summon ship is a first-level Life magic spell that costs two mana and causes a single ship to appear at the caster's present location on the Adventure Map.
I was stranded when they sunk my ship. At least the rain brought fresh water or I might have perished. Unfortunately, it took three weeks before someone found me. Never again!
On maps that make use of ships, this spell is so useful as to justify taking Basic Life Magic just to be able to cast it.
Although the description doesn't specifiy this, you must own a ship somewhere else on the map before this spell will work.
This spell is a favorite of map-makers who want to stop the player going over a particular sea before completing a certain section of the map. They disable this spell, and then have a "summon ship"-scroll as a reward for the quest when they want you to be able to cross the sea.