Mr Shuff, of the amazing 20kride site, pointed out an arcade machine by Sega that he used to adore – a sit down shooter set underwater, called The Ocean Hunter.  Apparently quite Steampunk, you and your companion (it could be two player) would travel the world in a hot air balloon, making your way around the globe to then dive into the depths and fight increasingly terrifying underwater horrors!

It was released in 1998, and I get the impression that whereever it was that Mr Shuff used to play it, is no longer available to him.  He misses it dearly, but took a rather blurry/liney video of the attract sequence on it – you can see it here, but I fear that it will not come close to conveying the feel of it.