Today was the busiest I've ever seen the beach area at Big Creek State Park. However, it was still a great experience because the swimming area is very wide and there's plenty of parking.
The marina was busy with people buying snacks and renting boats, but the line went quickly. We then easily found a shaded picnic table to eat our snacks at. It was a last minute decision to visit on a holiday weekend and I'm so glad we did.
Be sure to check Iowa State Parks' Beach Monitoring page before swimming.