The bug

If a player is steering a boat, a second passenger will only rotate client-side.

How to reproduce

1. Use the following command

   /summon boat ~ ~ ~ {Passengers:[{id:"armor_stand"}]}
   

2. Run the /data get entity command on the armor stand and have a look at its Rotation, it should be [0.0f,0.0f]

   /data get entity @e[type=minecraft:armor_stand,sort=nearest,limit=1]
   

3. Get in the boat and turn only very slightly
4. Run the /data get entity command on the armor stand and have a look at its Rotation

   /data get entity @e[type=minecraft:armor_stand,sort=nearest,limit=1]
   

   → It is still [0.0f,0.0f]

The reason

The following is based on a decompiled version of Minecraft 1.10 using MCP 9.30.

This happens because the method net.minecraft.entity.item.EntityBoat.controlBoat() which modifies the net.minecraft.entity.item.EntityBoat.deltaRotation value is only called client-side. It might make more sense for not steering passengers to use the rotation of the boat.

Note: At some point the method net.minecraft.entity.item.EntityBoat.applyYawToEntity(Entity) corrects the value.