TOPIC: Please help....

I can not know the meaning completely, please help

A.He is on the sea.
B.He is in the sea.
C.He is at the sea.
D.He is underwater.
E.She plays on the sea.
F.She plays at the sea.
G.She plays in the sea.
H.She plays underwater.

In A perhaps he is in a boat on the water (he is not on land).
In B perhaps he is swimming in the sea, not in a boat.
*C is not quite idiomatic; we usually say either
C-1 He is AT SEA.(=perhaps he is a sailor and his boat is at sea, not in port) OR C-2 He is AT THE SEASIDE.(=at the beach)
In D,he is under the surface of the water, perhaps swimming or searching for something in the water.