You cannot sleep underwater with a snorkel as the snorkel only allows you to breathe when the top of the tube is above water surface.
This means that you would not be able to get air when fully submerged and asleep.
To sleep underwater or be underwater for long periods of time you would need scuba diving equipment and oxygen to allow you to breathe underwater.
With a snorkel and flippers you can dive down to around 10 to 15 feet depending on your comfort level and breath holding ability.
For deeper dives you will require scuba gear and a tank of oxygen for breathing.
The greatest danger to snorkelers is drowning which can occur due to ocean surges, strong currents, shallow water blackout, inhaling water, cardiac arrest, being struck by a watercraft or colliding with a coral reef.
Shallow water blackout can occur when you hold your breath too long or resist the urge to come up for air.
It can lead to a lack of oxygen and a dangerous condition called hypoxic blackout.
Poor swimmers can snorkel as long as the poor swimmers wear a life jacket or snorkel vest and stay in shallow water with someone else.
Snorkels are not allowed in swimming pools because snorkels can be dangerous for swimmers that are inexperienced.
Snorkels when worn by inexperienced swimmers can increase the risk of shallow water blackout due to breath holding and can also obstruct a lifeguard's ability to quickly assess someone that is in distress.
The design of a snorkel can also make it difficult to clear the water from the tube if it fills up and potentially lead to water inhalation.
You can use snorkels in your private swimming pools but not public swimming pools.
Using a snorkel when swimming can cause people to hold their breath for extended periods and lead to shallow water blackout.
Shallow water blackout is a dangerous condition in which someone loses consciousness underwater even when they're are in shallow water.
Snorkels can also obscure your face and make it harder for lifeguards to monitor your condition and respond quickly to emergencies.
Some specialized swimming clubs and training programs may allow the use of snorkels under the supervision of a qualified swimming instructor.
And you often use specially designed snorkels to mitigate any safety concerns.