The Ergo Buoy from Aqua Sphere is a great training tool for working your core upper body when swimming and will immobilize your legs so that you can work on your upper body strength and technique. The ergonomic design fits comfortably between your legs and has a belt with quick release buckles to keep your legs together and prevent the buoy from slipping. Made from EVA ultra soft foam.
The buoy does work well in general, but unfortunately keeps working down your legs whilst swimming and belt slip out of unenclosed side during swim. I am now using with belt and ends detached.