It is a mixture. A pure substance contains only one type of substance or one species of any substance or element. Sea water is a mixture of different salts dissolved in water. Thus, it is not a pure substance. Another point is that the different components of sea water can be observed separately.
Salt .
salts
Chloride
It is a mixture of water, salts and oxygen.
The temperature(:
Likely gametes (eggs and sperm).
Coral??????????? Just guessing
Metals (such as aluminum) are held together by "metallic bonding" which can often be described as a "sea of electrons"
Practically any difference.
Practically any difference.
Mainly salts of different kinds.