No, you can’t control a GoPro by wifi underwater. That’s true even if it’s only the camera underwater and you (and the wifi remote or device) are above water.
Wifi waves don’t work at all well underwater. It might be possible one day to set up wifi networks underwater, but for now it’s not an option.
The problem is that the signal drops off very quickly in even a small amount of water. By the time you get a few inches, let alone a few feet, underwater, the signal is so weak as to be unusable.
So for all practical purposes, you can’t control your GoPro underwater with a wifi signal. That means you can’t use the Capture smartphone app, nor can you use the GoPro Smart Remote, the GoPro Remo Voice-Activate Remote (even though the Remo is waterproof), or the devices that turn an LCD Backpac into a wireless view and controller.
Jury-Rigged Hybrid Setups
If you’re looking to do some serious filming underwater and really need remote control, there are ways to hack together a DIY hybrid system. They involve sending the signal over a cable, so they’re not true wireless systems. They’re also rather involved to set up, so it’s not something you can just do on a whim. But if you want to try it, here’s a how-to guide.