Thus far, the only other situations shown to increase a bender's ability are:
- The full moon strengthens waterbending. This could arguably fall under the moon simply being out, but this is the pinnacle of that circumstance. (i.e. bloodbending can only be performed under a full moon)
- Water from the Spirit Oasis has unique properties that make it a powerful waterbending tool.
The Avatar Wiki says:
Furthermore, a Waterbender's power is strongest when he or she is situated in colder climates, during the winter, or near their native polar homelands.
I do not recall anything that explicitly demonstrates this in-show.