Isaac Bay was the highlight of my trip to St. Croix.
I drove to Point Udall to watch the sun rise, then drove a few hundred yards down the road to park at the trail head. There was broken glass in the parking area, so I'd recommend leaving your car unlocked and windows open (and obviously don't bring valuables with you and don't leave anything appetizing in the car).
The trail is well-marked, but lightly used, so it feels very wild and is a great introduction to the beach to come. It is an easy hike.
The beach is unbelievable - something you wouldn't believe still exists in this world. A half-mile of soft, white/pink sand, completely undeveloped, preserved for the turtles. I didn't see a soul for three hours, and felt undeserving of such a perfect place, all to myself.