One planet with a plausible ocean is Pluto. However, Europa, a moon of Jupiter, and Enceladus, a moon of Saturn, are the planets with seas that are the closest to the surface and hence the most reachable.
Ocean worlds' inhabitants
Scientists are interested in these seas not just because of how much liquid water they hold (the ocean off of Europe likely has twice as much water as all the oceans on Earth combined), but also because chemical reactions between rocks and the ocean water may be able to support life. In fact, it's possible that the climate in these oceans is extremely similar to the one on Earth when life first emerged.
In these situations, water that has seeped through the ocean floor's rock heats up and gets chemically enriched before being ejected back into the water body. This chemical energy can be used by microbes as food, which can then be consumed by larger organisms. Actually, neither sunshine nor atmosphere are required. Since their discovery in 1977, a large number of these heated, rocky formations, also known as "hydrothermal vents," have been identified on the ocean bottoms of Earth. In some places, chemosynthesis (energy from chemical reactions) rather than photosynthesis does in fact sustain the local food chain (energy from sunlight).
Tidal energy is primarily responsible for heating the stony innards of most ocean worlds in our solar system and keeping the waters from freezing all the way to the surface. This is in contrast to the Earth's inner warmth, which is primarily radioactive. However, the chemical reactions between water and rock are comparable.
By passing through ice crystal plumes that erupt from ice fractures, the Cassini probe has already taken samples of Enceladus' ocean. And there are expectations that when NASA's Europa Clipper mission starts a series of close flybys of Europa in 2030, it could locate comparable plumes to study. But entering the water to go exploring could provide far more useful information than simply smelling a freeze-dried sample.
In the swim
This is where the notion of independent micro-sensing (Swim) enters the picture. The plan is to use a radioactively heated probe to melt a 25 cm-wide hole through the ice at a location where the ice is relatively thin on Europa or Enceladus. This ocean is thought to be hundreds or thousands of meters below the surface.
Once there, it would let out up to four dozen wedge-shaped, 12 cm long microswimmers to explore. They wouldn't last nearly as long as the 3.6-meter-long autonomous underwater vehicle known as Boaty McBoatface, whose 2,000-kilometer range has already allowed it to cruise more than 100 kilometers beneath the Antarctic ice.
Independent micro-swimmers, deployed from a probe that has penetrated the ice crust of a moon. Not to scale. Image: Nasa/JPL