The Small Is Sufficient, But Tight
I have the small version of the Seal Line case. It does the job, is very handy (fits in my pocket), and I can still use the touch screen on my iPhone 4S - all great attributes. I use my case primarily for protecting my phone during runs/hikes in the rain and it has adequately done the job - I cannot attest to how well it would work if it was dropped in the water.
My reasoning for the four-star rating ("pretty cool") is due to the fact that my iPhone BARELY fits into it (with the case on). It is tight to the point of annoyance and is an effort to insert/remove my phone. It just seems counter-intuitive to me to produce a case that barely fits one of the most popular electronic devices on the market (to include iPod touches). Perhaps that is narrow-minded of me but I think it would have made more sense to make it a tad larger. If you have a smaller phone (or similarly-sized phone with a similar case or no case), it seems like it would do the job wonderfully. One could argue that I should have purchased the medium but then it would not have fit in my pocket.