Alright, so the sizing and fitting have already been addressed. Yet, it is so vital that this question needs to be addressed. When it comes to selecting the proper foot pocket, prior research needs to be done (but that was already addressed). If the manufacturer promises the foot pocket fits true to size, then go with your actual shoe size. If that happens to be an 11, then look for size 11; simple as that. Of course, remember that this accounts for bare feet or thin socks.
If you plan on wearing thicker socks for deeper dives, then you would possibly want to consider one size larger. The only issue with this is that excess room will require socks and if you decide to go out without socks, this excess room will waste your energy. Bottom line; really know your stuff before you buy.