From the Introduction:
To the best of our knowledge, the play-aid HYDRONAUT is absolutely unique and has the potential to be the most intriguing game you have ever played...and played...and played! The concept of Hydrospace exploration, which provides the setting for this unusual play-aid, has been almost ignored in the role-playing industry.
This playing-aid uses TRAVELLER with references to High Guard and the Journal.
The planet setting Dagan is non-canon. Includes a large full color world hex map and a one-sheet encounter tables insert.