What to use as suncreen for intense prolonged exposure?

I realize all the benefits from sun exposure, but there are still times when I am at the pool/beach, or out in the sun for over an hour during peak intensity that i know for sure I will burn. I have read that most sun screens contain carcinogens, and there are mixed opinions on natural oils (coconut, shea butter, etc.). What would be an effective non-carcinogenic natural, or store bought sun screen to use when you know burning will result otherwise?

http://www.iherb.com/Life-Extension-Enhanced-FernBlock-with-Sendara-30-Veggie-Caps/21214?at=0

The ashwaganda is used for a "14.5% reduction in cortisol levels in 98 chronically stressed individuals. This dose also resulted in a 13.2% increase in levels of the anti-aging hormone DHEA, which helps maintain the youthfulness of skin cells." If I'm not mistaken may also help with hyaluronic acid but not positive.

There is a cheaper option without the ashwaganda than the lifeextension but I can't find who makes it right now.

or you could take

Astaxanthin

Last year, I personally used high dose Krill before sun exposure and was happy with it. I believe happyman mentioned similiar. I believe it's said to last for about 4 hours and then you have to take more Astaxanthin/krill.

Shea Butter is about an SPF 4