I've been to Lighthouse point twice and while very pretty, I much prefer the Serenity Bay at Castaway over the one at Lighthouse Point.
Serenity Bay at Castaway is more secluded and quiet, being physically removed from the family beach.
It's all one beach at Lighthouse Point with only a small sign to designate the separation. Even at the furthest available umbrella I could still hear music from the bar area.
Also, the last time I was at Lighthouse Point, they weren't letting people put up the umbrellas due to wind. That's a big fail for my fair skin.