The problem was that during swim sets, my child and many of the kids were shivering. This long sleeve thermal swim shirt does the trick. My daughter is 9 and quite a string bean so I wasn''t sure on the size. I purchased the Medium for her. It seems a good fit. She says it feels a little big in the water though. If you want a less expensive option then try your local big box store. I bought a purple long sleeve shirt from their "active section" and it seems to work just as well for a fraction of the cost. Just make sure it is slightly close/snug to the body. Be sure it is that material that is best for workouts/fitness (similar fabric as a swim suit).  

Overall I''ll have to say we are very happy with the Finis Thermal shirt!!!  Great purchase if your considering it.

Great for swimming as I get cold quickly. Just wish it came in XS because it is a bit too big. Otherwise, awesome!

This shirt keeps me somewhat warmer in the chilly pool where I swim. It also creates a drag while swimming, for a better work out. Water works its way up into the upper sleeve, which creates the drag.

I absolutely love this shirt.
I have lost a lot of weight and am now very cold in the pool water.
This shirt helps fight off that chill and doesn't seem to slow me down.  If it is increasing "drag" and making me work harder, I can't tell and am happy for the extra workout.
My only complaint?  There are absolutely no care instructions and I would like to make it last a while.

Was very hesitant to buy this shirt at first because I didn't know if it would really be worth the money, but it goes down as one of the best purchases I've ever made in my life.  Decided to take this shirt with me on a "day" hike from the top to the bottom of the Narrows at Zion National Park.  Being early July, it was warm out and the Virgin River at the start is quite shallow, so I didn't start the hike with it on, but this shirt was on before my first dip into the water.  My hike wound up taking me 20 hours to complete, almost 10 hours of which were made in the dark and required crossing rapids and a little bit of swimming in the parts where I couldn't touch.  One of the best things I did was bring this shirt because it kept me warm, before and after I got it wet.  It dries really quickly!  I don't even want to imagine what this hike would have been like without it.  Definitely a life saver in that situation.  Shirt also helps me get into the pool a little quicker for lap swimming.  Heaviness of shirt while swimming is good for training, and doesn't create so much of a drag that form is lost.  Also good to wear on rafting trips.

It works as I use it to coach and teach swimming lessons in the water.  Keeps me warm, but the sizes run very, very small.  The XL I ordered feels like a Med.