You can use an automotive GPS. I have a Garmin Nuvi 200 GPS for the car with Garmin lake map and it was OK on the boat but the automotive GPS's do not have the ability to give great detail so are very limited. Also the screen size is rather small for use on the boat in my opinion.
I got a Garmin 521S (Sonar) GPS Chart plotter last year and Garmin Lake Winni map ($10.00 downloaded). This map of Winni is pretty good and along with the Bizer lake map (manditory in my opinion) it has been a valuable asset to our boating. If you do not have a depth finder on your boat you can add one to the base unit 521 or buy the 521S which has the sonar transducer included. Knowing the water depth is very important knowledge for any boat owner. The 521S is $550+ so it is a heafty investment but if you look around you can get them cheaper.
Try:
http://g.factoryoutletstore.com for great prices and service.
BTW the sonar transducers are made by Airmar in Milford, NH for Garmin.