I assume you mean the deck and hull are separating at the lip where the two are joined.
Is the seam fully separated? If so, I would insert wedges into the seam to hold it open. If possible, clean out the bonding surfaces using acetone. If you can scuff the two surfaces with a file or sand paper, that would be ideal. Then work (thickened) epoxy into the seam (it may be necessary to use a syringe). Remove the wedges so the seam closes and then clamp it together using moderate force. You probably want to place strips of wood between the hull and your clamps so that you have a nice, even distribution of the clamping pressure. Wipe away any epoxy that squeezes out of the seam and let the glue cure.