06 CRF450 drinking water?

Is there white smoke coming from the exhaust?

This might be a really stupid question, but does it have a catch can/overflow tank? My buddies 03 450r doesn't so you just have to keep topping it off as it boils over.

I've got a catch can and it isn't filling up. I'm going to run it tonight and see if there's any white smoke coming out the pipe. Would that indicate bad head gasket like it does with vehicles?

Yep. My 610 had what must have been a leaking head gasket for a while, then it finally blew. It used coolant, but never smoked. Then it blew, and definitely smoked! Hopefully it's something else, guess it's not a big deal to fix if your mechanically inclined.

The level will drop if you give it five minutes then you can fill it up again. Had the same "problem"

Better look at the oil and if milky (and don't see the above suggested indicators) suggest you look behind the impeller.

Years ago I had a CR250 that blew calcified water (yea, I know how'd the water not get replaced ? ) through the back of the impeller case and into the engine casing.

if the oil isn't milky just go ahead and order parts to rebuild the water pump. Seals, new shaft. If they haven't been changed since the bike was new, they're due anyway and quite likely they're leaking now.