Why do you go immediately for an Omega Seamaster? There are a thousand other waterproof watches out there with removable batteries that don't weigh near as much as a dive watch. It's rated for 300 meters. I don't think anyone is expecting a phone to be thin and light and waterproof under 300 meters of water.

Because it's well known enough. 200g is the heavier end, sure, but 100g seems pretty light for a wristwatch anyway. The Seiko 5 is one of the lightest watches at about 70 grams, and that thing is tiny. That's to say, the Seamaster isn't really much more heavier than others, unless you're going for some mickey mouse watches.