Since water reflects much more light (or UV for a sunburn) when the light hits the water at a low angle I though that it might be possible to get a sunburn faster in the evening than at solar noon if you are on a small boat in the middle of the ocean. Is this true? Are there any graphs showing how much radiation will reach you based on the Suns height if you are surrounded by water? Will the radiation that reaches you decrease as the Sun gets lower or is there a bump near sunset because of the increase in reflection?
No. The atmosphere provides significant shielding from UV. For a given time of day, being near the water will increase your UV exposure (because of the reflection); but as the sun gets lower, the decrease in UV content (because of atmospheric screening) offsets the increase due to greater reflection.
If you go up in altitude, it is considered that you will get 25% more sunlight for ascending about 2 km. If that means that the last 2 km of atmosphere filter out about 25% of UV-B radiation, you can see that "in the evening", when the sun is at an angle of say 70° to the vertical, the sunlight will have to traverse significantly more atmosphere (about 2.9x more). That means the UV content will be cut by roughly a factor 3x; much more than even perfect reflection from the water would add.
That doesn't mean you shouldn't be more careful on the water; just that you need to worry more about the time of day (when the sun is at its highest, it will be most powerful).