I don't think so it shouldn't be........
It depends upon the distance the water must travel to the sea.
Nothing is allowed to be thrown into the sea. Pollution is illegal. Exceptions made for clean water and some biodegradable substances. Plastic is a 'no no'.
They take water from the sea and extract the salt from it. Its an expensive process.
Desalination
No, but it is illegal to interfere with their operation
Boiled sea water removes chlorophyll from the plant which is essential for photosynthesis
No, it is not illegal but if you did not keep them in a seawater aquareum that was suited to their needs that would be cruel and there are laws which relate to the cruel treatment of animals.
take them out of water,
Cyanobacteria
Take a spoon and take some water from the sea, burn it, see how much salt there are then x 10000000000000000000000000000000000000000000000, you get the answer!
No because the salt water can take the pressure of the core under the earth, if there was a fresh water sea the water wouldn't be able to hold the pressure and instead of a sea it would be a gaping void in the earth.
Definitely not. The fish in the sea take in the dissolved oxygen in the sea thru their gills.