Iceberg

You conjure a 15 ft. by 15 ft. by 15 ft. cube of ice, which you can place floating on the surface of a body of water as long as no creatures occupy those squares and the water is sufficiently deep. The cube has 540 hit points, 5 AC, typical object characteristics such as immunity to poison and psychic damage, is made of fresh clean water, weighs roughly 96 tons, and floats.

While this can make a formidable obstacle, it was originally developed for transportation and shipping to float along the current for a day's worth of travel.

You may only have one instance of this spell active at a time; casting it again causes the first cube to disappear.