This could be a good idea for VIP items. It could open up a lot of new gameplay for Classic. Water mount sparring, water mount races, battles where your guild could go on a massive ship and everyone would have different rolls like cannons, steering the ship, e.c.t, you would have to take down another guilds ship using these mechanics.
OR
Instead of buying the 'boats', you could do the continuation of the scarf quest to get a 'free' one (this would be the only way to obtain a water mount), than, once you reach the town there could be 'water mount' shops, with various types of mounts to purchase for the water. This would be in addition to other buildings and shops in the town.