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Here's a good demonstration of what I know of as a "hydro mod" for a watch
It consists of filling a watch with an oil to give you a wider viewing angle underwater (due to refractive index stuff that I don't really know enough about optics to understand) and increased water resistance
https://www.youtube.com/watch?v=Bu4T4Cy4eKQ
It's pretty cool, and not too difficult to do
also, if you do fuck it up and have a bubble of air in there you can say "oh, that's intentional, it adds a spirit level feature!"