I wear my DD (almost 4 months) in a homemade ring sling at the pool. It's made from a blue madras plaid fabric that doesn't run when wet and provides good sun protection. I use the kangaroo carry and keep her cinched up very tightly and high on me so I can easily follow DS around the pool area. I also sit in the shallow water, watch DS, and DD gets her tush wet in the pool . She thoroughly enjoys it and because her entire body is in the sling, I don't have to worry about sun exposure, except on her face (she wears a big hat too). I don't know that you need a special sling for the water, just make sure the one you are using doesn't bleed when it gets wet, I guess, and keep baby cinched up tight in case you have to go running quickly after your older child. I take DS to Toddler Time at our local pool every week and it's worked out great for us with DD in the sling.
~~Kristina~~ Mama to DS(10/30/01), DD1(VBAC 3/28/04) and DD2(HBAC 5/21/06)
You could just buy 9 meters (or so) of solarveil, or athletic mesh, and hem the edges. If you were to do that, you'd want fabric that is stretchy, but not too stretchy, KWIM? I think the width of most wraps (Didymos, etc.) are about 20 inches, right? So if you got 60" wide fabic you could make 3 wraps! Gotta love that....
Jazminflower, solarveil is a fabric that blocks (at least partially) UV rays. http://www.solarveil.com/fabric.asp Taylor Made Treasures uses solarveil for water slings, and so do many others.
Sarah, Queen of Hearts, raising a Full House with Michael, King of my Heart! DS (2/02), DD (3/04), DS (1/06), DD (12/07), and DS (3/10) ~~*~~Not your typical Pastor's Wife!~~*~~