If it is Terminal 91 (I can't remember for Celebrity), I'd probably say go straight there from the airport, you're likely to be arriving at the pier pretty close to when they'd start allowing boarding (taking into account a 9:30 arrival and waiting for luggage).

However, while it may be possible to leave the ship after checking in, I'm not sure that the time frames you have will make it all that reasonable of an idea. I've done a 10:30 arrival with only a carry-on (which I think wouldn't be much later than a 9:30 with checked luggage), and while I didn't take the fastest route to the ship, there's no way I would have felt comfortable (or felt it was worth it) to leave the ship and go into town. (That day I took light rail into town, then a cab from downtown to the pier, saving me about $30-$40 on the cost of the trip from airport to pier, although it does take a little longer). But that terminal is far enough from downtown that the time involved getting there and back isn't negligible.