Thanks for your post!
In general, it makes the most sense to weight yourself at the same time of day - that way you can make comparisons more easily - you make a great point that weight fluctuates, primarily due to water intake.
This brings up the point that weighing yourself is associated with weight loss. Many studies have confirmed this. And interestingly, daily weights are more effective than weekly or monthly.
Take care!
Weighing yourself at the same time of the day is a good idea. For me personally I prefer early morning. I feel that provides a more accurate reading.
Weighing yourself after exercise is not a good idea. It's possible to lose considerable weight due to dehydration. If you are weighing yourself to see how much fluid you need to replace then that could be a useful recovery tool.