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Casio B640WB-1B
Cheap, durable, slim, not a flashy fashion accessory.
Battery will last for a decade.
Water proof, have used mine while metal detecting under water many times. Unlike leather or rubber straps this won't fall apart of you get cought on a branch or twig.
Being slim and black has several advantages. It won't EVER restrict your wrist movement, it will not make your wrist warm. It's also not super obvious to other people, so it won't be something NPCs would recognize in a police line-up.