There's a trick people use for it. You put on a wetsuit and fill your pockets with items they can't buy (like flowers or seashells). Then you go into the ocean for around 5 to 10 minutes (3DS should not be in sleep mode), and when you come back they'll start pinging.
The downside to that method is that they can also ping for catchphrases, greetings, and some other things. But it's a pretty reliable method.
The wetsuit method totally works. I just got a wetsuit myself recently, and in the past two hours, I've already gotten a pwp! A lot faster than normal. P: