Being on the southern coast (where the high winter waves don't occur as much as on the north shore), this beach is great all year!
The snorkeling is great here and the water is quiet and gentle because of a protective reef across the entrance to a small cove. There are actually 2 bay areas. One is more shallow and a favorite spot for families with smaller kids. That is also where the lifeguard station is set up. Close in on the other side, the fish are use to being fed, so they'll come check you out to see if you have food. Watch out that you don't accidentally get caught up in the shallow reef like I did---I ended up with one nasty cut on my hand! We came back to this beach many times, even though we were staying by Lihue. The ride was nice, and only about 25 minutes.
In the winter, daytime temperatures in Poipu range from the mid 70's to the lower 80's. Nighttime temperatures are in the low to mid 60's. In the summer, daytime temperatures go as high as the upper 80's, and during the night, down into the low 70's. But the water is a pretty constant temperature which makes it great for snorkeling in year around.
Poipu Beach was recently named America's #1 Beach by Dr. Steve Leatherman of Florida International University.