From a photographer's perspective, I would strongly suggest the 7:00 evening cruise that Waterton Shoreline Cruise Co. offers. The setting sun creates some interesting (and beautiful) photo ops of mountains along both the east and west shores of Upper Waterton Lake. Pay particular attention to the triangular-shaped north face of Mount Cleveland (southeast corner of the Lake), the rugged/jagged spires of the Citadel Peaks, and the almost-horn-like appearance of Campbell Mountain (west shore, just south of the International border). These are some of my faves to photograph at sunset.
The sun sets behind the mountains (as seen from your perspective on the tour boat) at about 8:10pm this time of the season. That means you will probably be at the Goat Haunt Ranger Station at that time. But obviously it is still light out, much later (daylight remains in the valley until about 10:15), so you will still get some great photos on the return trip back to Waterton townsite between 8:10 and 9:00pm.
As the moon is now beginning to wane, you will probably not have a chance to photograph the moon coming up over the mountains on the 7:00pm cruise next week, but keep your eyes trained to the south and east... You might still catch a glimpse of a waning crescent coming up over the mountain peaks.
Lastly, the 7:00pm cruise is usually the best time for seeing wildlife - especially if the day is hot and sunny. In my experience, I've seen more wildlife from the 7:00 cruise than any other trip of the day. This year already, I have seen a couple black bears, a moose and bald eagle from the tour boat after 7:00pm.
As a back-up... the 10am cruise is your second best option, as the rising sun makes for some nice views of the west shoreline of the lake, as well as really lights up the International border line as you look west.
I hope this helps! Let me know if you have any more questions!
Edited: 3:30 pm, July 05, 2012