Whitby 42 Brewer Sail Boats

Laume II is Hull #2 with a Perkins with the BW Velvet Touch tranny and a MaxiProp. The rear seal is shot, leaking about an ounce every two hours. Is it possible to change the seal in the water or does it have to come out? A diver tells me there is only about 3" of space behind the prop. If a diver can remove the prop, I would assume that the shaft can come back enough to clear the coupling, etc. Any experience with this process would be much appreciated

Thanks!

John