[partiallypro]

I was under the impression that the biggest contributor is actually just the rising temperature of the water, because water expands as it heats.

[Tomminn]

The average depth of the ocean is about 4km, so a thermal expansion of 1x10^(-4) per degree C (right order of magnitude), would result in about a 4m rise per degree increase in average ocean temperature. Which is obviously significant.

One of the issues with this theory is that the average temperature of the ocean is below 4 deg C, and in this regime the coefficient of expansion is negative. So we might actually see a small sea level drop due to ocean warming.

[fullarr]

While there is probably a slight amount of expansion in liquid water from heat, water expands most when frozen solid. this is the entire reason ice floats

[partiallypro]

Half of all recent sea level rises have been from thermal expansion of water: https://www.nationalgeographic.com/environment/global-warmin...