Why did I still get sunburned? Is this normal?

I'm 17, and I have fair skin. Today I went on this two hour boat cruise on a river. It was in the 70s and mostly cloudy. I used sunscreen on my arms and neck with SPF 50, and in the morning I used facial moisturizer that has SPF 15, so I thought I'd be ok. My arms didn't get burned, but my face got burned pretty badly, along with the top of my head where my hair parts. I find that kind of ridiculous, because it wasn't even really sunny and I had SPF 15 on my face. Is it normal that I still got sunburned, or could something be wrong? Is there some kind of vitamin deficiency that causes you to get sunburned really easily?