Hi LittleFish,
Good choice, I love diving the Cove at night. I wouldn't suggest leaving your gear on the grass though. It works for day dives, but not for night. A night dive at the Cove means you are gearing up at and returning to your car / truck.
A couple of things to think about for a night dive at the Cove vs a day dive. First, Lifeguards will be gone, so plan accordingly for any emergencies that may crop up. Depending on what night you go, parking may be an issue. The Cove is a popular spot for evening strolls, romantic liaisons and local dining, so parking is most difficult up to about an hour after sunset, and then it starts opening up. I've always been able to find a spot, but I usually get there early because I know I'm going to have to wait for one to open up.
The entry / exit to the Cove is best accomplished by a "line of sight" path in line with the main stairs next to the lifeguard tower. This helps you avoid the partially submerged rocks and reefs that stick out from the cave on your left as you look at the water, and the rock cliffs on your right. It's easy to see the water's reaction to the rocks / reefs during the daytime, but at night, rely on the lineup.
Other than that, dive safe, have a great time and enjoy your night dive at the Cove!