"Porcupinefish are medium to large sized fish, and are found in shallow temperate and tropical seas worldwide. A few species are found much further out from shore, where they can occur in large shoals of thousands of individuals. They are generally slow swimmers.
Porcupinefish have the ability to inflate their body by swallowing water (or air) and become round like a ball. This increase in size (almost double vertically) reduces the range of potential predators to those with much bigger mouths.
A second defense mechanism is provided by the sharp spines, which radiate outwards when the fish is inflated. Some species are poisonous, having a tetrodotoxin in their skin and/or intestines. As a result porcupinefish have few predators, although adults are sometimes preyed upon by sharks and orcas. Juveniles are also preyed on by tuna and dolphins."
This reminds me of an old idea, which I never tried ... partially inflate a mat with water, and then add some air. The mat would be heavier and easier to duck dive. How it would ride, however, is a mystery!