Aquatic mounts confer an increase in swim speed, but cannot be used at all on land. There are very few in-game and, as of Legion, there are only five permanent ones, two related to Vashj'ir zone, one related to Darkmoon Faire (added in Patch 7.1), and a couple from the Broken Isles. Aquatic mounts do not give their riders the ability to breathe underwater, so plan accordingly with one of appropriate items or abilities.