When you get to St Davids you need to head out of the town towards Fishguard then fork left straight away to go towards Whitesands. The rugby club is on that fork in the road. Follow signs for Whitesands when you get there park in car park and instead of heading towards beach you will see a path that heads off towards the headland to the right of the beach. Follow this path for quite a way. There are various paths leading down to various outcrops all pretty good marks for spinning and obviously the further you walk the water tends to get deeper.
Streetmap: http://www.streetmap.co.uk/map.srf?X=173403&Y=227483&A=Y&Z=115
Google Maps: http://maps.google.co.uk/?ie=UTF8&ll=51.896718,-5.295458&spn=0.002373,0.009012&t=h&z=18