I've tried Rain-X periodically for years and have never found it to be worth much. On the boat it does nothing EXCEPT if we are getting heavy spray in rough water and IF the Rain-X coating is brand new. Then the water sheets off the windshield very nicely.
But in light spray, fog mist, and the light rain we get a lot here Rain-X accomplishes nothing because the individual droplets are not heavy enough to slide down the windshield by themselves. Rain-X does make it easier for the wipers to clear them, however.
The big problem with Rain-X that we've found is that in a salt water environment it doesn't last very long. Maybe for one run into the islands out and back if we're getting salt spray on the windows. I don't know if it's the presence of the salt or what, but after a day of spray and clearing it with the wipers, the Rain-X is pretty ineffective after that.
We occasionally use it on our vehicles where the higher speed of the vehicle helps clear the water. But even then we've not been impressed with it. Even following the application instructions to the letter and polishing it with a clean cloth, it still increases glare at night and makes streaks.
Up here the only way we've found to effectively clear the light mist from fog, light rain, and light spry is wipers. If you boat in heavy rain I suppose Rain-X would help for awhile. But even when Rain-X is working as advertised, you have all this water running down the windshield which doesn't obstruct your "big picture" vision but can make it very difficult to see debris in the water, which we have a ton of up here from crab pot floats to boat-sinking deadheads. The only sure way to see those things is with the windshield panes absolutely clear and the only way we've found to do that effectively is with good wipers.
GBs typically have very good wipers--- ours does and it's an ancient '73 boat--- but we found that the wiper blade arc in the most critical pane, the center one, didn't come down quite far enough for my wife to see the water directly in front of us when she was steering or helping watch for debris in the water. Fortunately one of the Grand Banks forum members who lives in Seattle came up with a brilliant way of installing a parallel arm wiper on a GB without having to drill any holes. He made us the part he had designed so now we have a longer parallel arm wiper on the center windshield pane which makes a huge difference. The stock wipers are fine on the two outboard panes.
But a lot of people swear by Rain-X including the owner of the BMW shop we use. He says he never turns his wipers on at all, although every time I've ridden with him or he's followed me in his car in the rain, he's turned his wipers on . But I would definitely give it a try rather than taking anyone's word for it. Your boat's windshield configuration, the kind of water on the glass you have to deal with, and other variables may make it a good solution for you.* We always drive our boat from the lower helm, never from up above, and in our waters good visibility, especially right up close to the boat is critical. We have found the only consistent and effective solution is good wipers.