Will Grayl filter sea water?

Short answer, No. While the Grayl (and almost any other water filter) can be used with water from almost any source, it can not be used with saltwater. If you try, I promise your water will still be salty, you'll be less hydrated than before, and your cartridge won't be happy.

Salt water contains ions like magnesium and sodium that our filters are not effective against. To remove salt from ocean water, you need a desalinization filter.