Well it would sure be hard to move on land with those fins of theirs, right?
coral
It is very hard to breathe underwater without assistance from an air tank.
Some lizards like crocodiles, Birds, possibly some dinosaurs, some turtles,
There are hard-shelled & soft-shelled. There are those that live in fresh water and those that live in salt water. Hope that helps. And there are those that are big enough to bite your finger off and those that aren't. hehe
There are many such hard shelled insects, mostly beetles.
What exactly do you mean by "hard shelled?"
A hard shelled sea creature is the barnacle.
Platypus eggs are not hard-shelled, like birds' eggs. They are soft-shelled and leathery.
The shell covering the top of a turtle's body is called the carapace (say karra-pace).It is made of a hard substance called scutes.Some turtles do not have a hard carapace. The carapace of soft-shelled turtles and the leatherback turtle is made of thick skin.The shape of a carapace is not always the same:
bugs
If you are underwater it is difficult to breathe. This is saying that this is a hard time in your life. Your head is under water. Your having trouble staying afloat.