This is not so for parts of the globe. Indeed, since insolation is distributed by latitude, one would expect that net radiation would also vary by latitude. However, over the years low and high latitudes experience the same average temperatures, i.e. they don't progressively heat or cool. This situation can only be possible if energy is continuously transported from equatorial and tropical areas into middle and high latitudes. As a generalization, land heats and cools faster than water, and to a greater degree.
Water has a higher specific heat than land.
Specific heat - amount of energy needed to raise of 1 g of substance by 1 C. Water is a much better transmitter than land. Water bodies have unlimited supply of water for evaporation. Evaporation cools the surface of a water body. Because of these properties, water acts as a reservoir of heat. Just like insolation and net radiation, temperature is generally distributed by latitude.