I've had a couple occasions where I needed to see something under-water with a remote camera. The first thing that came to mind was my GoPro. It would be perfect for this type of thing, but the Wi-Fi only has a range of a few cm under salt water, so rather than viewing live on my phone I had to drop the GoPro in on a pole and watch the recording later... Then I had an idea...
Because water (especially salt water) attenuates or absorbs Wi-Fi signals, it is not possible to remotely operate a GoPro camera with more than about 10cm (of fresh water) between the camera and remote device without a wired connection between the two. So a wired connection is exactly what I've done here, but it's simpler than you think (It was simpler than I thought anyway!).
Mission: Locate 9 year old's lost toy boat in 4 meters of cloudy salt water (without getting wet)
Equipment: 5m RG6 TV antenna coax, plastic pipe or pole of your choice, GoPro with Wi-Fi, Phone.
Simply strip some of the shield from the coax to expose 26mm of center core as shown. Glue the coax to the rear of the waterproof GoPro case with 2 part epoxy or silicon adhesive. At the other end just hold the exposed coax near your phone. Why 26mm and not ~31mm? Well, I looked at a WiFi router antenna I had lying around and that's what it had inside the rubber casing. I could have spent all day working out the optimum antenna length to suit the coax and RF coupling arrangement, but I'm lazy and it worked with 100% signal strength on my phone! I assume that the coax I used would attenuate the signal too, but again, it works.
Note that it is important to seal the coax against the back of the GoPro case because you don't want water getting up the cable or between the coax and case. The Wi-Fi signal will pass through the case and glue.
Another use for this arrangement would be to use the Wi-Fi remote under water when diving. If the Go-Pro was located away from the remote, a length of coax could be externally coupled between the camera and remote. Again both ends should be sealed.
Result: With the GoPro cable tied to a 6m length of 20mm plastic water pipe, and the coaxial cable fitted as shown, I was able to view a live feed and control the GoPro Hero 3 at 4m depth in salt water with 100% Wi-Fi signal strength. I suspect I could double that length. Did I find the lost toy? Unfortunately not, although if I had, I would have had to go for a swim!