The pop-out floats on the S61 were installed to improve the lateral stability of the aircraft in the event of a ditching. As you note, the S61 is designed and approved for amphibious operations.
Due to the high C of G on the helicopter they are prone to capsizing in anything above a moderate sea state (Rotor shut-down in 3 foot seas has been demonstrated). These floats were initially designed for use with the small sponsons installed on the SH-3 (military S61) series utilised by the US Navy.
S-61 floats were mandated by the CAA in the late 70's/early 80's and were retrofitted to all commercial S61's along with some other very practical modifications learnt from previous accidents/incidents.
If you look
here you will see a very interesting image of a ditched S61. Unfortunately, in the picture you cannot see that the top of the cockpit is missing, following a blade strike on landing that significantly altered the cockpit arrangement!
The aircraft was flown off the rig and ditched in the water, where, unable to shut it down (no throttles, power, etc.) it was abandoned and cruised around the rig uncommanded, for some quite considerable time before running out of fuel. Fortunately it never contacted anything else out there!
Hence you have the very unusual image of an S61 running on the sea, in what would have to be described as a pretty reasonable swell.
After it ran out of fuel, it sank!