I have a merc 350 mag and this is the extractor that I use.
http://www.boatersworld.com/product/366982973.htm It has an adapter that screws on to the merc dip stick hole. Don't get an electric pump... I had one and it was horrible.
Definitely heat up your engine before you suck out the oil, otherwise it will take an hour to get the oil out. I change the oil around every 50 hours because it is so simple using this extractor.