Any ice below sea-level will melt into water that will take up exactly the same amount of space. So any ice below sea-level that melts will not cause sea levels to rise.
I learned that ice has a (slightly) lower density than water, hence it usually floats. I was convinced that submerged ice is doing so because of an additional weight on top of it (above-water ice pack).
Maybe there is also ice that is submerged without this extra weight, but then its density should be lower than water.
Either way, I don't see how submerged ice could take up exactly the same space when melted. If it has a different density, it should change in volume (elementary physics).
When that ice melts, it still weighs the same amount and fills exactly the amount of space that it was displacing. In other words, it fits perfectly in the space in the water that it was already occupying.
So floating ice that melts (above or below the water) doesn't change sea levels at all.
Only melting ice that is currently supported by land will increase sea level.