I have been watching this thread get rather technical the poor original poster must think he has joined a science/physics forum.
I used to have a Avon 580 RIB in Torquay Marina for a couple of years that had no cover, so consequently collected a fair bit of rain.
Over the first 6 months I had a flat battery about 4 times. I then decided to fit a 8 watt solar panel that supposedly is not powerful enough to overcharge a battery but sufficient to keep the battery charged up. After fitting this I never had a flat battery again.
The bilge pump was a Rule 750 connected through a float switch so when the pump was not working there was no battery drain.
How does a boat get spray inside when it is kept in a marina?
With regards to the technical answers I did not see any calculations for evaporation !!!
Keep it simple