Depends on the boat and the windshield.... The more compound curve, the harder they are are to do. Plus, some dashboards get so close to the windshield they can't get in between them.
That being said, the new Malibus are very easy to do. I just had mine done for $120. Took him an hour. Didn't need to remove the windshield or anything. Those corner pieces on the Malibu windshields eliminates that nasty curve that's hard to work with....