There is really only one proximate cause of a rising sea level. Rising sea levels are caused by increasing temperatures.
As the average temperature of the Earth as a whole rises, sea levels must rise as well. The reason for this is that higher temperatures cause more of the Earth's ice pack (mostly at the areas surrounding the North and South Poles) to melt. That water flows into the sea and raises its level.
The cause of the rising temperature (in the opinion of most scientists) is the increasing amount of greenhouse gasses in the atmosphere.
The increase in temperatures includes an increase in water temperature, which accelerates the melting of ice caps. What causes the rise in sea level though is not so much the melting of the north pole ice, but of the ice from Antarctica and Greenland. These are actual land masses with huge masses of ice on top of them. When they melt, the water flows into the ocean and adds to the amount already there. The floating mass of the north polar ice cap whether it is solid or liquid, does not directly affect sea level.
Global warming is one of the reasons the ice caps are melting. The ocean's water is becoming warmer as is our global temperature. The temperature of our planet is rising due to the burning of fossil fuels, the cutting down of trees, and the development of land.
I agree with the previous responders. Ice caps are melting and this is causing the sea levels to rise, no doubt about it. This is also directly linked to global warming. According to the EPA, sea levels have risen approximately 6-8 inches in the last 100 years.