Since this is in trend I will start this one.
1. Using water moves on them can put them off guard, especially if you use rain dance and then Thunder them to death.
2. Sending them a bulbasaur for a friend and then having him use leech seed on them really gets on their nerves.
3. Saying that you never liked to swim and then releasing them into the ocean.
4. Catching them just so they can subdue the fire types so you can catch them and then releasing the water types into the wild via PC.
5. Planting a very nasty plant in their back.
6. Laughing at their face when they lose to a Raichu that you made them fight.
7. Accuse them of not training hard enough to overcome their weaknesses to Thunder and Grass types.
8. If you calmly explain to them that there are about 500 other water types out there that are probably much better as a species than they are.