Did you know there was an official Black Lagoon x Seiko collaboration? There was! They're pretty nice.
I actually own one! This isn't mine, but I have this edition that came on a different strap.
However! The manga/show itself has tons of meticulously chosen guns and cars, but no watches. I for sure thought that when Revy goes diving in the submarine arc there'd at least be a general shape of dive watch to me work with, but nope--they went swimming without watch timers. Bold! Given that Black Lagoon is set sort of nebulously in the 1990s, I wanted to pick something that'd be a little aged and beaten up already by that point.
This classic Seiko diver seems like something that would be pretty easy to find, durable enough for a mercenar--uh, shipping company that spends a lot of time in and around the water. Wearing it on a nylon strap seemed appropriate for the humidity in Roanapur. They weren't super expensive but not cheap either--affordable, but maybe she took it off a dead guy or won it or something. Rock would've spent way too much of his old salary on a Rolex Oyster Perpetual that Revy thinks makes him look stupid.