Hydronaut was published in 1981 by Group One as a 48-page book with a large color map.
Reception
William A. Barton reviewed Hydronaut in The Space Gamer No. 46. Barton commented that "if you don't mind fiddling around with some areas and making up your own data to cover what is missing, you mightfindthe ideabehindHydronauts worthy of your attention – especially if you're one of those refs who doesn't always have the time necessary to create your own adventures from scratch. If you haven't liked G1's past efforts, though, you probably won't like this one, either."