A recent study in the Journal of Strength and Conditioning Research tested flotation restricted environmental stimulation (R.E.S.T.) with a 1 hr epsom salt bath in a dark room after heavy eccentric leg extension. The results: blood lactate and perceived pain were greatly reduced compared to control.
Flotation in a zero stimulus environment allows the muscles to completely relax. The addition of epsom salt to the water contributes by increasing the availability of magnesium to the muscles, furthering relaxation. This increases blood flow, the clearing of lactic acid and gets the recovery process off to a good start.
In addition to the benefits at the muscular level, the increases in magnesium will also aid in better sleep.