The classic Spinward Marches map uses blue for "water present" and white for "no water present." At first I'd assumed that "no water present" simply meant Hydrographics=0. But comparing this output to maps of the Regina subsector didn't produce the right results - there were many more "no water present" worlds.
I asked on the TML and did some poking around, to no avail - the Atmosphere code had some effect, but it wasn't obvious or documented, so I added it to my "TODO" list. Looking at TD #6's map of the Shuna subsector of Lishun I was inspired to check the rules again - Book 3 and MegaTraveller Referee's Handbook. And there it was.
So, for reference, water is present if:
- Hydrographics > 0 (duh)
- Atmosphere > 1 (else, ice capped)
- Atmosphere < A (else, exotic, so a fluid hydrosphere if Hydro > 0)
That seems to work.