Just want to report that after a bit of fiddling and investigation that the Gulfman seems spot on with the moon phase and tides both at current time and scroll ahead date and time.
I think the most important thing is to determine as accurately as you can the lunatidal interval for your location. Longitude can be found on a google search for your area.
You can also search the internet for lunitidal locations near your area then fine tune to match the location you specifically want.
I have recently bought a mudman and gulfman at the same time so offer a few thoughts for those wanting to buy either or both.
Recessed protected face.
Titanium screws and back
Wide band suits thickness of case
Stopwatch reset as button A
Red 'G' on bezel
Very comfortable back and strap, the best of the G shocks that I have tried
Tough buttons which feel secure and sealed
Long timing stopwatches
Like the proportions of the watch, was worried it would be too big but smaller than my PRG-40
Red buttons are either love or hate gray might have been better
Time movement graphic hard to see / waste of space
Flash alert graphic area could have been an lcd icon like mute etc, waste of space
Gulfman seems a bit more conservative than the mudman but love them both they are better than I had expected.