A non-flush hatch secured with two or three straps screwed into solid wood sheer clamps should stay put through even very heavy surf. One key is fastening the hatch down tightly enough so that water can't lift an edge.

I just got back from the Newfound Rendevous, where I saw a lot of beautiful flush hatches. How do those builders feel about the security of their hatches in surf? Although there is nothing for the water to easily grab onto, the hatch is held on with an internal bungie which seems to me to allow more potential movement than external straps.