I know this topic is old, but I thought I'd offer up the answers anyway.
Barchetta does mean "little boat", and was the name given to the first open two seater 166MM. The Touring body styling was unique in that the sides tapered inward at the sills, which made the body "boat" shaped.
Berlinetta was the name reserved for the sporting GT coupes. In the early days, these were typically raced. The last of the true road/race GT's was, of course, the 250GT SWB (short wheelbase berlinetta). Cars after this were still called berlinettas (250 GT/L Lusso Berlinetta, 275GTB, 365GTB/4, etc), but they weren't expected to be raced.