Hi Jamie. The sea level mainly raises due to ice from land getting mixed up in the sea. For example, at Antarctica, the ice sheets are a part of the land mass and not the sea. Due to global warming, this ice sheets break off and run into the ocean, thereby displacing water equal to its mass (Archimedes principle). This is what causes the ocean levels to raise primarily.
Now let’s take a look at the icebergs that are already in the ocean. When these icebergs melt they shouldn’t displace any more of the water they had already displaced. This is only true when the water from the ice has the same concentration as of the sea water.
But if the icebergs are made up of fresh water, a different process happens. The sea water is a little denser when compared to the fresh water. So when the fresh water mixes up in the ocean, it takes a little more volume than its frozen form, thus raising the ocean levels slightly.