in your last paragraph. They aren't totally waterproof, I guess
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The technology around the button shaft seals is as per nearly all shaft seals - an o-ring. G-shocks have tiny o-rings around their button shafts and having pressed G-shock buttons repeatedly at 45+....I can't see how it's a problem.
Where I think the dive computers have an advantage is in their size. With a bigger case, you can more easily separate compartments so a flooded battery hatch (using Suunto as an example) doesn't flood anything else.
But most importantly, you can use bigger o-rings and you can put more support around the shaft, which should minimise any axial distortion of the o-ring - this is what the Hublot guys are warning against when they say not to press buttons underwater - sideways movement of the shaft distorting the seal, not the button depression breaking a seal.
The real problem is the risk. My G-shock, though sentimental, wouldn't dent the wallet, a flooded Spido would.