I had the 3M Crystalline put in my car (went on the inside). I think it is a great product. It comes in a variety of tints, from almost clear to dark. While the darker tints are a little better for UV and heat rejection, not much. I think the tint is more about aesthetics and privacy.
Here in Texas
, you're not aupposed to tint your windshield (absent a medical
condition) so I didn't. But I did do the rear glass, about the same angle as the front. On a hot summer day, the dash under the windshield can get too hot to touch. Under the rear window, it gets warm, but not even hot. The difference is remarkable.
It was expensive, about $500, including installation
redsky49's point is important. Glass is a much higher percentage of the skin of the cabin
of a car than the glass on the boat is. Given the heat pouring in through the other surfaces, you might not notice the effect of tinting the glass.
But, tinting the glass will block the UV, and that might greatly increase the life of the surfaces and fabrics that the UV will otherwise destroy.