Given the intolerance to moisture of any kind that cell phones have I would go with a good dry bag, Ziplocs sometimes don't seal completely. If you don't need your cell on the water I'd leave it behind, besides trying to make a call on it when there is any amount of wind is going to be interesting.
Northwest River Supply, REI, etc. carry watertight phone cases (~$30).
I use a small dry bag clipped in the storage cubby and have a couple of general purpose watertight pouches with criticals like my ID carried on my person, in my PFD pockets.