Why do greenhouse gases affect ocean temperature so much?
No, greenhouse gases do not absorb infrared radiation from the sun... the Earth is really the source of infrared.  The amount of infrared energy from the sun that reaches Earth is insignificant.  Visible light from the sun heats the Earth, NOT infrared light.  The visible light passes through the atmosphere and is absorbed by the surface of the Earth.  Then, the Earth re-radiates that energy back into the atmosphere in the form of heat (aka infrared radiation).  The infrared radiates back into space, but if there are infrared absorbers in the atmosphere, the energy is not lost to space.  That has a net warming effect.  In fact the Earth needs greenhouse gases, or we would all be frozen.  It is a delicate balance between GHG concentrations and climate effects on the hydrologic cycle.

It is important to note that ocean water expands in response to temperature increase... so even just a modest increase in ocean temperature can lead to significant sea level changes.  Human civilization is thus directly affected by oceanic temperature increase due to sea-level rise.

As a side note, the absorption of carbon into the ocean is really of more environmental concern than the actual heating of the ocean, in terms of acidic effects on the marine ecosystems.  The ocean absorbs roughly half the carbon emitted into the atmosphere and there are areas already seeing the negative effects of acidification.