Closed cell will hold water over time as well but thats the foam you want.I would suggest laying a sort of false floor down low in your hull before applying to keep foam off the bottom to reduce the chance of water retention.I know a guy that filled under the floor with 2ltr pop bottles after he removed soaked foam,don't know the floatation value of the bottles but a different idea anyway.My last boat had no exposed foam below the gunnel,anything below was glassed into sealed compartments to prevent water logging.It can add 100's of pounds to your hull weight if exposed to water over time,even the closed cell.