Most likely leaking from the seal. 590 hours is not that much time but that would be over 12 years. And you run salt water through it as opposed to mine which is only fresh water.
If you are mechanically inclined or know someone that is , you can purchase the re-build kit.
For example Torrenson
http://shop.torresen.com/marine_die...i2&ident=308435&spnum=K28270-42000&name=Water Pump Rebuild Kit, 2GM, 3GM, 2GM20, 3GM30, 2GML, 3GMLE&image=yl.gif&multi=5
has the kit for under $100.00.
A new pump will be $400 plus.
You may not need the whole kit if your bearings are still good and there is no play. That site above also gives you a price for individual parts.
I scanned the attachment
Good Luck