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good for 300m just by putting oil in it... Why doesn't someone just do that in the factory?
They did. Bell and Ross had one called the Hydromax. Rated to 11,100 metres but only 39 mm. https://uniquewatchguide.com/bell-and-ross-hydromax-dive-watch/ I've got one sitting in a drawer. Battery changing has to be done at the factory, because of the oil. I think they adapted it from a Sinn. Sinn have had at least 2 oil filled dive watches, including the UX and the Hydro.
Pff, pretty sure I've seen dudes on YouTube making a water resistant watch good for 300m just by putting oil in it... Why doesn't someone just do that in the factory? Probably best as something with wireless charging, I guess, but still.