The beach in Mirissa usually means the strip down Beach Access Road, but the water near the Palm Villa Hotel is also very nice. This beach, guarded by an awesome mini-island which you can climb, is safe to swim in, though there are some rocks randomly about.
The beach is nice to swim in, but a key feature is the little island in the water. You can walk up to it and up the top (there's stairs). There a bit of seating here and there, but it's mainly a nice view. Note that the side away from the Palm Villa is completely unswimmable and dangerous, at least near the rock itself.
Anyways, coming back to the beach, it's like a pool. There's sea turtles there quite often as well, so good for snorkeling. Only guests at the Palm Villa can use the day beds, but it's a nice place to sit and have a beer or food.
Walk up to the top of the rock and have a look around.