I live close to that area and the temperature in early April is not terribly predictable. It is "usually" warm, capris or shorts during the day, but you may need a light sweater late evenings or in the early mornings. Normally I have needed a sweater for the outdoor sunrise Easter services.
We certainly don't swim in the ocean or in an outdoor pool in early April, but Myrtle Beach has several visitors who do. Normally, when we see folks swimming at that time of year, we assume they must be from a much colder climate.
If you wanted to be safe, I would suggest as someone else already has, that you get a hotel with an indoor and an outdoor pool. But other outdoor activities should be pleasant at that time of year. Hope you have a great time.