I thought it was because the water might develop swells due to maneuvering to get in and out of port. Obviously sometimes that wouldn't happen, but it might be policy just in case.
As far as at night, could be cleaning, but I thought it was for safety reasons as well. If the water gets choppy or swell-y the water can start slopping around FAST. It is very dangerous to be in a pool if the ship starts rocking- it wouldn't take much to be swept over the deck.
Just My Humble GUESS.