Living on a lake, I leave my 1999 23' Excalibur on the Boat lift all summer. It is needing an oil change (20w-50 Mobil 1).
I am getting a bunch of work done to my trailer and will not get it back for a couple of weeks.
I am trying to figure out a good way to change the oil on the lift.
I have not found a decent oil changing pump locally.
I know some of you must have taken this challenge in the past and have some good ideas.
A friend suggested draining it through the transom drain plug into a bucket and washing the bottom out with solvent. I just know my nieghbors would have a heart attack if they saw this.

Here's the best
http://www.tatpumps.com/

From your name and posting, I'm assuming you have a '99 502 Mag?
Peice of cake then. Merc makes a hand pump that screws onto the top of your dipstick. That's why there are 'thread-looking' things at the top of it.
Screw on the pump, and pump away. A very clean process. It will take just a minute or 2 of pumping. Change it after the motor has been ran, it will make it easier to pump out.
It can be bought from a Merc dealer, but it is pricey $60?? or an aftermarket one from West Marine for $40
The dipstick tube goes all the way to the bottom of the pan, so most of the oil will be sucked out.

I personelly prefer the hose attachement to the drain plug. You've got to spring for the electric oil changer apprx $120 bucks and works like a charm.