I went last year and had a uncomfortable night since I couldn't get close in due to the construction works. At present am planning to go to Pontrieux for Bastillle Day, so may go back to St Cast and try the new Marina afterwards.
Access is easy with port and starboard buoys leading the way in, we were met by a dory and directed to a finger.
The pontoons are all in place and they were just finishing of the last bit of dredging. The shore facilities are a long way from being complete, just porta-cabins at present.
St Cast itself is a pleasant sea-side holiday town, main disadvantage is that it is long walk, 15 mins + from the marina to town to get bread in the morning.
Didn't make it myself, got to Paimpol, and was planning to hop along the coast on the way home but with only a couple of days in a week that looked suitable for getting home had to head straight for Guernsey, maybe next year, by then the facilities might be finished.
We just came back and thought it was fantastic, easy entrance, 24 hour tidal access, and don't worry about the walk into town, I doubt its as long as 15 minutes, and there is a wonderful lit walkway below the cliffs into the town
My kids loved the beach(es), we found a great restaurant in the town (Les Halles) and the cafe in the marina was good also. The walk around the headland from the marina is also worth doing.
Should be a must on any cruise of the area - we'd go here rather than St Malo next time