Thought about this some more, if you are keeping the boat on a trailer it's even easier than being in a slip. Run the boat, if it drips too much, tighten the packing nut. If you still aren't happy, pull the boat out on the trailer to replace the packing. Trying to replace the packing while in the water could shake you up a bit the first time, it's doable but you will be happier on the trailer
