Boy, I hate it for you. These things are always in the most miserably inaccessible of locations, it seems.
I replaced one a couple of years ago. I budgeted a couple of hours' time for it as a do-it-yourself job the morning before I hoped to launch the boat. The experienced yard manager swallowed hard and readjusted his launch schedule after I told him this.
It took me about 16 hours of hard, contorted, sweaty, miserable work to back the end of the propeller
shaft out of the coupling so I could get the old seal off. The yard manager said they usually had mechanics put in no more than two hours trying to do this. If it didn't come out quickly and easily (and it usually didn't), they would then take a Sawzall, cut the shaft, pull the whole thing out, and have a new shaft made. This was cheaper than continuing to fight it at mechanics' labor rates.
After I got mine apart, I paid the yard mechanic
two hours' labor to put the new seal in and reseat the shaft. This was no piece of cake either.
But if that seal blows, you've got a real problem on your hands. I guess you'd better go on and fix it. The seal itself is not expensive. The labor charges are a real danger
, though, unless you are dealing with an honest, sensible yard.