We stayed at Topsail in April a couple of years ago. It was a typical weather year and we had a week of sunshine, with a couple of overnight showers as well as one really foggy morning (which made for some great pictures down at the dunes as the sun was coming up).
Temperatures were moderate and a windbreaker was needed in the morning and evening. Mid-day was very comfortable on the beach but we enjoyed kite flying more than getting in the water, it was too cold for our taste. The teenagers would wade in up to their thighs.
But, it is a huge beach and you can literally walk for miles with a variety of scenery.