The GoPro HERO4 Silver has very limited WiFi range when used with the GoPro Capture app for the iPhone. If your camera is underwater, even a few inches, the WiFi connection will be lost. I was even having difficulty maintaining a WiFi connection when my camera and iPhone were separated by a wall/windows. The GoPro “Smart Remote” is advertised to have a range of 600 feet. The main disadvantage of the Smart Remote is you can not see what you are filming.
I found this video on YouTube that describes how to extend the WiFi connection between a WiFi enabled GoPro camera and a smartphone. Many Thanks to Jan Siimson for doing this. Please be sure to check out his YouTube Channel.
I tested this idea “out of water” at about 70 feet using some RG-58 A/U coax cable I had in my shop. Put my GoPro Hero4 Silver outside on my driveway. I closed the garage door, continued running the coax into the house, closing the door to the garage, then out a sliding door to our deck, closing the sliding door. Worked perfectly! This solves a major issue of controlling my camera when it is outside our house from my iPhone that is inside.
I then purchase 50 feet of RG 174 U type coax (50 Ohm) and redid my “out of water” test. Worked perfectly. I then took my kayak to a local lake here in Salem Oregon and tested out your idea using the RG 174 U coax in 10 feet of water and it worked perfectly. Will try in the deepest part of this lake soon, which is about 25 feet.
Cam-Do sells a “GoPro Underwater WiFi Extension” setup in different lengths and their 60-foot system is $257 (USD). To be fair, it should be noted that Cam-Do makes and sells very high-end industrial strength products.
In the image above I am using small zip-tie anchors to hold the end of the 6cm stripped end of the 50-foot length of RG 174 U coax cable. This setup works perfectly and allows me to securely anchor the coax cable but still be able to open the waterproof housing. According to this “Tear Down” for the HERO4 Silver, the WiFi antenna is located above the lens. On my iPhone-5s the Wifi antenna runs along the top of the phone on the left-hand side.
I am guessing this setup would also work with the GoPro Smart Remote for underwater control of your WiFi enabled GoPro camera.