Firstly, when the water warms, it expands, which makes sea levels rise. Secondly, melting land ice and snow due to higher global temperatures adds more water into reservoirs. Higher sea levels endanger all the coastal areas, causing population residing there either to migrate or spend countless money on the investment in structures protecting the cities from rising water.
Notice the distinction between the effects caused by melting sea ice and melting land ice. Sea ice does not affect water volume in the oceans, because it already drifts there. However, land ice, when melted and leaked into the oceans, directly increase water volume.
Another thing to pay attention to, that global average water temperature is a mean of temperatures at different locations. That’s why in some places water can be even 3 °C warmer, while in others, it can happen to cool down. The heat is not distributed equally in the entire ocean, and it also gets affected by events such as El Nino.
You can view the ocean-related data presented in the graph forms by following the link in the topic materials section.