Sunset Beach is one of the best beaches in Oahu for swimming during the summer months. The water is calm and a clear blue that you won't be able to believe. This beach is on the north side of the island.
It may sound silly since Hawaii is synonymous with warm water surfing but it does get chilly sometimes. You won't need to pack your winter 4/3 full suit by any means but a nice 1 or 2mm wetsuit top will help keep your core warm and allow you to surf longer, especially if the trade winds start to pick up.
You get wet. It's like a shower. Of course, if the water fall is too large… the weight of the water hurt you, can knock you to the ground, smash you into rocks, push you into the water, damage your eyes, make it hard or impossible to breath, etc….