If you’re like many sailboat owners, you like a little musical accompaniment on board. But most small and mid-size sailboats, particularly older boats, lack the equipment. Well, digital electronics resolved this problem several years back with the introduction of mp3 players like Apple’s iPod. (PS tested the iPod and wrote about its performance earlier this year; see PS February 2006 “Ocean Tested.”)Mp3 players have amazing capacity, and their sound quality is generally very good. The main drawback is that these units aren’t water-resistant, let alone waterproof, so some sort of case to protect them from the elements is in order.…