Best advice is to avoid anchorages that are exposed to a north swell even if they are calm when you tie up in late afternoon (CGB and White Bay, JVD are notorious and from the cruising guides & chart briefings this should be common knowledge). With the multiple sources of marine forecasts available at our fingertips there is plenty of advance notice available, and the swell can sometimes very quickly build in just a few hours. There are several good close-by alternatives to CGB and White Bay if a north swell is forecast - Great Harbour, Little Harbour, and Diamond Cay at JVD, and Sopers Hole.

If you are moored with a heavy swell, at the very least don't leave the vessel unattended and head to the beach! I recall a few years ago the motoryacht that disintegrated on the rocks at White Bay, JVD. There was heavy swell and the mooring pendant snapped as the ~20 ton boat pitched up. The vessel was ashore within a minute or two.

If the swell unexpectedly picks up at night and you're stuck (or you get a serious squall), I would be in the cockpit maintaining a watch. I might then set the anchor alarm with a short radius of 1 boat length to help as an additional 'set of eyes' especially if it was a very dark or rainy night. I would not rely solely on the alarm and head off to bed. If the mooring fails, by the time you wake up to hear the alarm beeping, jump up on deck, and assess the situation, it may be too late.