Sink is the act of submerging under water. Like when an object thrown in water weighs more than the water it displaces, it sinks down.
So, we can have a sinking ship, a sunk boat, a sunk box etc.
Drown means suffocating under water and dying. Thus, is a person jumps in water and doesn’t know swimming, he would be sinking. If someone saves him and pulls him to the shore, he will not drown. However, if no one helps him, he will be suffocated under water and die (one word – drown).
1) The boat sank, 15 passengers drowned.
2) Suresh is in the ICU. He is sinking (Note: Sinking here means his condition is deteriorating).
3) Everyone thought Sudha was drowned when she emerged from under the water and jumped up.
4) This company will sink if something big is not done soon