Dang that summer ice! It's always scratching my boat. :rofl:
Anyway...
Karl is right. The seals aren't what I would call a 100% duty type. a 99 sportster will have the carbon seal system, and they are a "wet" seal. They are designed to let a little water past to keep them cool. Since you have 2 of them... and if they are worn at all... you could have a boat sitting on the bottom by morning. Not to mention, you could have water work it's way into the pumps. (bad)
Or even worse... you could have a boat that is still floating... but the engines are half under water. (insurance won't take care of that)
If it were me... I would take it out of the water at the end of the day.
Just an FYI... My boat was completely rebuilt at the beginning of summer last year, so the seals are new. After a day of playing... there is probably 3 or 4 gallons of water that comes out of the drain plugs. That still isn't enough to hit the bottom of the bilge pump. Who knows how much there would be overnight.