I am not a scientist but the way thermodynamics works is that hot usually always seeks cold, not the other way around. Heat is always trying to radiate to the atmosphere. Some things act as heat sinks but that is another issue. To hopefully answer you, if volcanic activity were to warm the ocean, the nearest depths would heat first, or absorb the heat since they are colder than magma. Now if, say, 100 meters of ocean above the volcanic activity warmed, the ocean above that would be colder. Now the the slightly warmer layer transfers that heat to the water above. So on and so forth. By the time the heat reaches the surface it would be dissipated in theory. Unless the source of heat is way way hotter than a volcanic vent and massive. 
 

hope this helps.