I haven't done that in ages! They're called "Bain Flottants" here (floating baths), and they leave you nice and relaxed and your skin feels fantastic, to boot. They look like giant eggs - something right out of Mork and Mindy.
No worries, Isa. You can keep the lid open, plus the sheer volume of salt keeps you easily afloat and your head above the water. You can put your arm in back of your head if that makes you feel more secure, or else they somtimes provide a little pillow. Just watch the water in your eyes. It stings worse that ocean water.