Enceladus, the sixth-largest moon of Saturn, is probably unique in being the only body in our solar system besides Earth (and possibly Europa) to have significant amounts of liquid water. Most of this water is contained in vast oceans underneath a thick layer of ice on the surface, and is kept warm by the planet's geologic activity. As such, it has been proposed as a potential place that could harbor life. Irregardless of that debate (of whether there is life on Enceladus), are there any Earth-based lifeforms known that could hypothetically survive in Enceladus's oceans?
Probably the closest thing to that environment on Earth would be Lake Vostok, which is 4 kilometers below the antarctic surface. Very little, if any, light gets down there, and it's really really cold, but still liquid.
Scientists recently found bacteria living there, so it stands to reason that bacteria would survive on Enceladus, but the really amazing thing is that the bacteria are ones that go with fish and crustaceans and mollusks, which would indicate that there are actually molluscs, crustaceans and even fish living in Lake Vostok!
From the BBC article:
A large number of bacterial sequences, reports the team, were from "animal commensals, mutualists and pathogens… including those associated with annelids, sea anemones, brachiopods, tardigrades and fish."
Also discovered: eukaryotes and archaea.
Depending on exactly what temperature the water on Enceladus is, the answer is yes. Bacteria are an obvious choice. But there is a higher lifeform which can almost certainly survive:
Tardigrades (Water bears) can survive for days at −200 °C - longer at higher temperatures, and can cope with extremely acid or alkaline environments. They can even survive in outer space.
The probable water temperature on Enceladus is much higher than this, and while there will be dissolved salts, this should not be a problem to the hardy tardigrade.
There actually just was a scishow video about this, where they said, that a specific kind of bacteria can survive in a simulated Enceladus ocean in the lab.
A recent and open access paper in Nature Biological methane production under putative Enceladus-like conditions describes an in-depth analysis of sources of energy in Enceladus' ocean and suggests that molecular hydrogen as H2 may be present and "eatable" by some organisms currently doing similar things here on Earth, e.g. Methanogenic forms of the Archaea domain single-celled microorganisms. From the introduction:
The most prominent potential source of H2 in Enceladus’ interior may be oxidation of native and ferrous iron in the course of serpentinization of olivine in the chondritic core. Olivine hydrolysis at low temperatures is a key process for sustaining chemolithoautotrophic life on Earth9 and if H2 is produced in significant amounts on Enceladus, then it could also serve as a substrate for biological CH4 production.
You can read more about serpenization in the answers to the question What is serpentinization, in the context of disappearance of surface water on Mars?
The detection of silica-rich dust particles, as an indication for ongoing hydrothermal activity, and the presence of water and organic molecules in the plume of Enceladus, have made Saturn’s icy moon a hot spot in the search for potential extraterrestrial life. Methanogenic archaea are among the organisms that could potentially thrive under the predicted conditions on Enceladus, considering that both molecular hydrogen (H2) and methane (CH4) have been detected in the plume. Here we show that a methanogenic archaeon, Methanothermococcus okinawensis, can produce CH4 under physicochemical conditions extrapolated for Enceladus. Up to 72% carbon dioxide to CH4 conversion is reached at 50 bar in the presence of potential inhibitors. Furthermore, kinetic and thermodynamic computations of low-temperature serpentinization indicate that there may be sufficient H2 gas production to serve as a substrate for CH4 production on Enceladus. We conclude that some of the CH4 detected in the plume of Enceladus might, in principle, be produced by methanogens.