Technically, steel wool is not a good choice for use on boats as it tends to leave tiny particles which will later rust and leave little spots under your finish. Granted, there are still folks who use it, but be aware of the potential for problems later. They do make bronze wool, if you can find it, but it may be a lot easier to arm yourself with a batch of Scotchbrite-type pads with assorted coarseness. One good source for such stuff is a tool rental place like they have at most Home Depot stores. You can buy big disks of the stuff designed for rental floor sanders and cut chunks out with scissors as needed to use by hand.
There are also people who strip old varnish using flat steel scrapers (carefully sharpened) and do a nice job of it, but I haven't tried it yet.