Winter (October to May) it's gonna be a 5/3 (air temp 8 to -2°C) and booties. Summer (July to September) you will be okay with a 3mm wetty. Surfing all year round nearby.
M4 then A48 then A40 to Haverfordwest. Then take B4341 will take you to Broad Haven. Park in the carpark next to the beach. Be careful as the water comes right up so don't leave any kite on the beach on an incoming tide.
During summer be careful as there will be many people on the beach. If it's slightly offshore think about going to Newgale beach just up the road or down to Dale a few miles away.