Endermen, striders, snow golems, and blazes are not hurt by water if they are riding a boat. Interestingly, they do take damage when wet in minecarts.

What I expected to happen was...:
Water-vulnerable mobs would take damage to water, even while in a boat.

What actually happened was...:
Water-vulnerable mobs do not take damage.

Steps to Reproduce:
1. Create a two-block-deep pit that is large enough to fit a boat.

2. Place signs along the floor of the pit (so the boat does not eject its passenger).

3. Place a boat with your favorite water-vulnerable mob amongst the signs.

4. Place a water source on the upper layer of the pit and observe that the water touches the water-vulnerable mob, but does not cause it to take damage.