Because of the salt - salt is denser than water.
salt water
Cold water is more dense.
On land fresh, on sailboats salt water.
salt water
fresh watar
salt water is more dense than tap water
Salt water will be more dense than the water, but less dense than the salt.
Salt
The density of liquids really depends on the suspended matter in it. If you have really dense concentration of salt in water, yes it could be denser than an egg. If you have merely a weak solution of salt, then the egg is denser... it depend on how much salt you have in the water.
Sea water is more dense because the minerals, especially salt, make it more dense.
Things float in water if they are less dense than the water they are floating in. Putting salt in the water makes it more dense, so things that are a little more dense than ordinary water float in salt water.
The salt makes the water more dense.