Tow floats are an essential item if you intend to swim in open water.
There are a couple of different types of tow floats.
Standard Tow float - Just used for visibility, these tow floats have no internal storage so you can't use them to carry keys / phone etc
Tow Float / Dry Bags - These are designed to allow you to carry your phone, keys and even clothes with you while swimming. These tow floats are ideal if you are swimming somewhere you need to carry your belongings with you.
Tow Donuts - These have a small ziplock bag on the top of float where you can keep smaller items such as keys / phone and are accessible while swimming.