There are a great many studies targeted at the effects on the environment of nuclear war on land, but I can't seem to find any relating specifically to nuclear war at sea.
The current basic setup in this world is that a genetically engineered alien slave race is attempting to invade the Earth via a portal on top of the Hawaiian islands. They were engineered specifically to be the best possible land fighters, but are woefully inadequate at any other region of fighting. Despite this, their technology makes them incredibly resilient to modern arms, and the conflict quickly went nuclear. Humanity has managed to hold the race at a stalemate on the ocean for some seven years now.
The primary nuclear armament of the human armies are in the 20-100 kiloton range, though about 100 multi-megaton sized nuclear weapons have been used as well. The human forces have detonated about a dozen nuclear weapons per day for about 6 years. What would be the environmental effects caused by this war? Would the increased water vapor in the air cause a global warming effect, or would there be a global cooling effect?