You can use the /ride command while in spectator mode. You can do things like ride and move horses, minecarts, and boats.

I don't expect this is intended, as a player in spectator mode isn't supposed to be able to interact with the world.

As this is done with a command, I'm not sure how much of an issue this is. Even as a spectator (with command permissions) you can still interact with the world, with a /setblock command for instance.

Unrelated to world interaction, running /ride while spectating an entity preserves the spectating view while also acting as if you are riding the entity. You can steer a boat while spectating it, for instance.

If you are spectating a boat and then run a command to ride a horse, you are not taken out of the spectating view of the boat but can still make the horse jump. The horse also moves as you move your mouse. You cannot move the horse.