lower unit grease looks watered down!!!!!

I just changed the fluid in the lower unit, ran it about 3-4 hours and it looks like it is watered down. What would cause this other than the seal behind the prop(looks ok) and is this a costly repair. Also the seals are in place on the fill and drain screws. Thanks,

What kind of oil ya put in it? http://www.bassboatcentral.com/smileys/popcorn.gif

Have it pressure tested. It will tell you quick. Could be an upper driveshaft seal.