Bonded Mount

The creature is magically bound to a beast with an innate swimming speed trained to serve as its mount. While mounted on this beast, the creature gains the beast’s senses and ability to breathe underwater. The bonded mount obeys the creature’s commands. If its mount dies, the creature can train a new beast to serve as its bonded mount, a process requiring a month. (Dark Tide Knight, pp. 205, PotA)