Because the inside of the glass is not shaded, 20% on a boat will look like 35% on a vehicle. Not that dark. But it is definitely not safe to go darker. Even 20% can impede vision.
Mostly asthetic. You will notice a difference on items that are set up on the dash and under the windshield, like a phone for example.
The gap mentioned above is because the elevated dash keeps the tinter from trimming the film perimeter in certain places with 90% access. There may be a gap to fill or they may cut into the gasket to avoid the gap (which isn't acceptable). In some cases, the only remedy in getting a near perfect job is to remove and reinstall the glass.
Releasing the film backing and applying it in an unprotected area makes a clean job nearly impossible.
It's difficult to shrink the film and get the film to adhere to a curved windshield. A compound curve is really tough.
Before having it done discuss some of these issues with the shop.