I changed the oil pan on my Mercruiser I/O (302 Ford motor) for the same reason, but I was stubborn and young enough to do it in the boat. I basically laid on my stomach and removed the bolts from the pan. Not an easy task since the salt water had rusted the heads badly. I used three different size sockets:SAE, Whitworth, and Metric in order to get a good bite on the heads. Had to drive most of them on. I placed a mirror in the bilge so I could see what I was working on instead of being totally blind. Having said all this, I would pull the motor.
As a note, if you are planning on keeping the boat a while, try to locate a stainless oil pan. If not, use a very good coating on the new pan and use stainless bolts and lockers