I fail to see the practicality of tinting boat windshields. It's not like a car where you're shielded from the direct sunlight on the interior and your pupils can account for it. Your pupils will be really small due to all of the ambient light and it will look just as dark from the driver's perspective as it would from in front of the windshield.
There are only 2 times when it would be beneficial 1) photo ops/ hanging out trying to look cool and 2) right at sunrise and sunset while traveling towards the sun (It would likely be a negative when going away from the sun at those times)
Also, I don't think you'd get the look you're expecting when about the same amount of light is hitting both sides of the glass. If you've ever seen two rooms separated by a two way mirror that effect is dependent on the mirrored side being lit more brightly than the observer's side. If you crank up the lights on the observer's side the effect is almost completely lost. Same in a car which is darker than the outside almost all of the time accept at night with the dome light on. So unless you go obscenely dark with the tint (Defeating the purpose of the glass) I don't think you'll get much of a mirrored look.