Casio and G-Shock watches with ‘hydro mod’ survive 4,000+ meter depths

“Hydro-modding” has been getting some renewed attention recently. The simple process involves opening a quartz watch case, filling it with oil, (usually mineral or silicone oil), and then closing it. With an oil-filled display, the light will not refract when viewing it underwater at an angle, whereas a non-modded watch will become unviewable with a mirror effect when viewed underwater past certain angles. Having a case filled with oil also makes the watch able to withstand more pressure and improves the water resistance. Sinn diving watches that have an oil-filled case, such as the U50 Hydro and UX, have a certified water resistance rating of 5,000 meters.

While hydro-modding has been frequently featured and discussed online in the past, people have only recently started sharing depth tests of their modded watches. In the past year, people have been pressure-testing their hydro-modded watches to amazing results, including real world tests.

Benjamin Lowry (@submersiblewrist on, who frequently shares images of military divers wearing G-Shock watches, shared the story of his friend Joshua Konicki who attached a hydro-modded Casio F91W to a ROV Deep Drone that was then sent to 4,950 meters deep in the ocean, with images showing the watch still intact and operating at that depth. What makes this even more impressive is that the F91W only has a “Water Resistant” rating with no depth rating. That rating means that it is meant to resist rain and splashes but is not meant to swim with, although we’re sure many people do swim with their non-modded F91W watches without any issues.

YouTube channel Defender previously did a real world depth test with a hydro-modded F91W and dropped it to 200 meters alongside a non-modded watch. The modded watch easily survived and remained working while the non-modded watch expectedly suffered a cracked display. The video also shows how to do the mod and states that you can use baby oil.


Most recently, The Dropzone Channel used a pressure chamber to simulate deep ocean depths and test modded and non-modded G-Shock GA-2100 watches. The video also shows a clear example of how hydro-modding greatly enhances the visibility of the watch display underwater. With the non-modded GA-2100, the minute hand stopped working at around 400 meters, and the LCD display cracked at around 750 meters. The mineral glass display cracked at around 2,000 meters. With the modded GA-2100, the watch went to 4,300 meters without issues.


Based on these tests, it’s clear that hydro-modding a watch can greatly increase its water resistance, but there are risks and possible drawbacks, and it will void your warranty. Some possible issues are sensor interference, oil volume changes due to temperature fluctuations, and reduced battery life on analog models. We suggest doing more research before trying it, or maybe just starting with a F91W.


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