Water can pass through the skin; they have lungs
amphibians
Yes aquatic life does need light and oxygen
Water does have sufficient dissolved oxygen to sustain aquatic life. This is true because aquatic life has adapted to the limited oxygen using gills.
Whales are aquatic mammals, not amphibians.
Some organisms are not adapted to live in a saline environmement.
Many aquatic animals do.Some amphibians also do.
lay shelled eggs
Turtles are adapted to an aquatic life, spending almost all their life at sea. They only come to land to lay their eggs - which need to be laid out of water so they can develop.
no. a hippopotamus is a semi aquatic mammal.
Amphibians
Neither. Turtles are (aquatic) reptiles and penguins are (aquatic) flightless birds.
the skin of mammals is not water permeable like amphibians skin is. Amphibians need to be either in water or moist environments at most time or else they would just dry up. Another reason is that mos amphibians have a larval stage (ex.tadpoles) that can never leave the water. This basically bounds amphibians to water. Amphibians also have webbed limbs adapted for water and not land.