Not quite true.
Light is absorbed in the first hundreds of meters, most in the top 10 meters. This light warms the water-air interface where evaporation happens. You are confusing warming the entire ocean with warming the surface layer.
There are segments of the ocean that are warmed from below and have no significant thermal gradient in consequence. Example: the Red Sea is located in a rift valley with massive quantities of geothermal heat entering from the bottom.