What sort of numpty would even suggest the condom solution, with a knot in the end ???
For flat water, go for the "Glad Bag" put the phone on speaker mode, and you can listen to music, and hear the birds singing !!
Matter of fact, the birds are sining on dusk here in Torquay right now !

(I found a Kathmadu top with this pocket on the upper arm.... I saying to myself, "what this pocket for?"... I tried it for coins, but they just rattled.... then I saw a jogger with an iPod strapped to their arm.... Stupid me put 2 & 2 together, and it is a great pocket for the iPhone4 on Speaker Music Mode on iTunes, in a "glad bag"



!!)