IIRC, reflection of the sun off water is 5% when directly overhead to about 65% when at a glancing angle (low on the horizon). I prefer to close my eyes and aim my face at the sun for about 10 minutes a day if I'm working indoors all day plus whatever incidental sunlight I get. I have SAD during the winter months and use a full-spectrum lamp, then.

People who spend more time in the sun have a low-moderate risk of melanoma, but higher risk of other skin cancers, vs those who spend more time indoors having a lower risk of non-melanoma skin cancer and a moderate-higher risk of melanoma cancer.