Depends on which iPhone you have.
Here is a very reasonably priced waterproof case for the 6 that I use constantly. I take my phone out racing with us everytime now that I'm confident with using waterproof cases. Add a $10 Bluetooth speaker and we've got tunes all night during Thursday night racing. This case works great, but I wouldn't intentionally submerge my phone with it. I did the papertowel test when I first took it out of the package, left it fully submerged overnight and not a drop of water. But, the phone is too expensive to intentionally submerge it, in my opinion. $36 iphone 6 waterproof case
Here is a $20 case for the iphone 5/5S. This one I probably would trust for taking underwater pictures with my old iphone 5. Loved this case and used it for a good while before I upgraded to the 6. Wish they still made cases like this for the 6 at a reasonable price.http://www.amazon.com/gp/product/B00B59N...arch_detailpage
Keep in mind most, if not all, of these cases are only rated to 6 or 7 feet of depth. I highly recommend picking one up just to protect your phone if you use it frequently around the water or other harsh environments. I personally wouldn't use one to try and turn my iphone (unless you have an older model that's just sitting in a drawer anyway) into a gopro.
They do (pretty much) all come with lanyards, so you could easily attach a float if you wanted.