I don't know about constant, but temperature is moderated (buffered) by water. Water has a high specific heat, meaning that energy enters and leaves with relatively little change in temperature. read more
Conclusion - the amount of water on earth decreases during the Southern Hemisphere summer due to oceanic photosynthesis, and then increases again in the Southern Hemisphere winter. Secondly, I believe there must be a significant long term change in total water amount due to fossil fuels being used. read more