With a tamed Horse if you right click a boat whilst it is on the leash/whip type item it will pop up with 'LShift to Dismount' whilst on the boat.
MC-13709
When a tamed Horse is on a leash and you go in a Boat you can LShift out the Boat
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I believe that now all entities you can ride on (Horses, pigs, boats and minecarts) requires use of sneak key to dismount it