From the free dictionary here are two terms that are often confused.
1. An environment capable of absorbing heat from an object with which it is in thermal contact without a phase change or an appreciable change in temperature.
2. A protective device that absorbs and dissipates the excess heat generated by a system. (my bold)
A thermal reservoir, a short-form of thermal energy reservoir, or thermal bath is a thermodynamic system with a heat capacity that is large enough that when it is in thermal contact with another system of interest or its environment, its temperature remains effectively constant. It is an effectively infinite pool of thermal energy at a given, constant temperature. The temperature of the reservoir does not change irrespective of whether heat is added or extracted. As it can act as a source and sink of heat it is often also referred to a heat reservoir or heat bath.
Lakes, oceans and rivers often serve as thermal reservoirs in geophysical processes, such as the weather. In atmospheric science, large air masses in the atmosphere often function as thermal reservoirs.
The oceans are often incorrectly called heat sinks by many involved in the climate change debate. They are more appropriately called a heat reservoir since the heat capacity of the oceans is about 1000 times greater than the atmosphere and the average temperature of the upper ocean is greater than the atmosphere. There are also two phase changes that can be expected with the oceans, fusion or freezing near the poles and evaporation away from the poles. I tend to cringe every time the term Heat Sink is used knowing any observations that follow are most likely flawed.
The large thermal capacity of the oceans and lakes is not contained at their surface but rather at considerable depth. The most stable temperature in the oceans for example is the 4 C thermocline at an average depth of around 1000 meters. This is below the majority of surface generated mixing which is the reason for its stability. Properties of water including the maximum density of fresh water at 4C degrees and the temperature of fusion ranging from 0C to around -2 C for salt water with around 35 grams of salt per liter of volume help maintain a fairly stable reference temperature. In deeper fresh water lakes the 4 C density prevents them from freezing solid by allowing less dense ice to form on the surface insulating the more dense 4 C sub-surface liquid water.
Depending on the source, the oceans contain about 1.5 x 10^9 cubic kilometers of water. Each cubic kilometer of water contains one billion cubic meters (10^9), with each cubic meter containing 1000 (10^3) liters each containing 1000 (10^3) grams. That would be a total of approximately 1.5 x 10^24 grams of water with an average temperature of about 4 C degrees.
How much heat is contained in that volume is a bit confusing due to the phase change potential. If it all evaporates it would have to gain 2260 Joules per gram plus about 4.2 joules per degree increase in temperature to the evaporation point which could be lower than the standard boiling point. For all of it to freeze it would have to release 334 Joules per gram plus about 4.2 Joules per gram as the temperature reduced to possibly -18 C for a saturated salt brine. A fair SWAG for the effective total heat capacity for the oceans at 4 C average would be about 334 Joules per gram which happens to be equal to the effective radiant energy of a surface at 4 C degrees using the Stefan-Boltzman relationship for an ideal black body.
Typically, the specific heat capacity is used to determine how much energy would be required to change a volume of a substance by 1 C degree which for a gram of water would be about 4.2 Joules. To raise the entire global oceans by 1 C would require about 4.2(Joules/gram)*1.5 x 10^24 grams or 6.3 x 10^24 Joules assuming no change in the rate of evaporation or freezing takes place in the process, which is unlikely for larger changes in temperature.
The SWAG estimate of total heat capacity may be relevant to basic Greenhouse Effect considerations since the ocean reservoir effective heat capacity of 334 Joules per gram happens to equal the best estimate of the effective Down Welling Long Wave Radiation (DWLR) felt at the "surface" of the Earth. DWLR is a bit unique to atmospheric physics. Most other branches of physics consider the net heat flux not the difficult to manage gross heat fluxes. While the SWAG may not have an obvious physical meaning, it does at least provide a stable, as in thermal reservoir, frame of reference.
Hopefully some will find this bit of minutia useful. If you happen to notice any calculation errors, please do not hesitate to comment.