When I first got my boat, the delivery team noted the exact problem you had - a lot of water coming in when the engine was in gear. The engine had 1000 hours and Volvo recommends replacement of their seal at 500 hours (though I now know these things last a lot longer than this), so I bought a new one.
Turns out the old one was fine. There are two 'hose clamp' screws on the aft portion of the seal - they were loose. I had to do a few other things to the boat, but I would have been pretty angry if they'd had to take the shaft out just for something that wasn't broken. Sounds silly, but it might be worth a moment to check on those clamp screws if you haven't already; obviously the delivery team hadn't on mine. The seals are relatively bullet-proof if lubed once a year.
