About as far south and west as you can get in Wales, right at the tip of Pembrokeshire, Martin's Haven is just inside the Skomer Marine Nature Reserve.
As with most shore dives, I like to get below the surface as soon as I leave the beach and follow the large pebbles out slightly east of north, biasing my course away from the jetty and towards the right hand side of the bay. One of the marine reserve rules is that divers should keep away from the jetty area and the left side of the bay while the Dale Princess ferry to Skomer Island is operating.
Kelp and pebbles eventually give way to a silty sand seabed just before the first mooring buoy. Following this round to the east brings you to terraces of rocks rising to the cliffs at the side of the bay. Kelp grows on top of the rocks, but there are enough steep sides and overhangs for plenty of other marine life to get a look in.
The rocks just aren't exposed enough for the solid walls of anemones found at many sites round Skomer island. Here there are occasional anemones and small dead men's fingers, cup corals, burrowing sea cucumbers, tunicates and bryozoans.
At the right time of year it is a great place for nudibranch spotting. It is not unusual for nudibranch hunters to spot five or more different species on a single dive.
Further out off the point, the seabed is a coarser sand, swept by the current running along the coastline of St Brides Bay. Sand dwelling critters include anemones, tube worms and scallops, but don't be tempted to take one for the pot as you are still inside the marine reserve. Divers must not take anything from the sea except for tidying obvious litter.
Navigation back to the beach is a matter of following the join between sand and kelp covered rocks round to the first mooring line, then following the slope up to the south and slightly towards the jetty to end up in the middle of the beach.
It is not unusual to get seals in the bay, particularly late in the afternoon, and sometimes they want to play. I can remember watching from the beach as a couple of friends towed their SMB back in, a young seal popping up and down alongside it. It was not until they surfaced that they realised what was happening, so they went back down and the seal played until their air ran out.
Having warned about obstructing the ferry and not diving the left (west) side of the bay, I have to admit that the point off the left side of the bay is the better dive. A much bigger reef descends down to 25 metres or so, with holes big enough for conger eels and lobsters.
The safe way to do it is when the ferry is not running; when they have a day off, while the ferry is out on an extended run, or after it has finished for the day. If in doubt just ask the boatmen and they will advise. Chances are that there are more opportunities to dive this side of the bay in the winter.
For training exercises, the boat moorings make convenient reference lines for practising alternate air source ascents and controlled buoyant lifts. Again, it is best to check with the boatmen that they will not be wanting to tie off while you are blobbing up and down.