Proline

If your worried about acid washing and cost, you can apply a coat of Star Brite Marine Polish with Teflon (avail. @ West Marine) to the underside of the hull in the spring. It makes removal of the build up very easy and the acid treatment is needed. It is of course an additional help to rub down the underside a couple times per summer. My boat has been in the lake for years and the underside is in showroom condition. Another product people like is called Easy On which supposedly inhibits fresh water growth.