djs has already given you everything you need to know insruction wise. I will say though that there's not really much money to be made in selling solely boat/rv coverage (unless of course you're located in a wealthy area near the cost - that may have a lot of yachts). Most of the time people insure their boats/rv's with their regular personal lines agent. If you really want to expand and you're going to go through the work to get your P&C lincense, the best way to become profitable besides strategically buying other agencies to expand, is to pick a (preferably) commercial lines specialty, learn the coverages in and out and sell to your audience - this will usually yield nice premiums.
