Despite numerous weapons being created during Star Wars' long, sprawling history, lightsabers remain the most iconic. However, while they can slice through just about everything — other than Beskar — can they avoid being snuffed out in the vacuum of space or in the depths of the ocean?
By comparing real-world physics to what transpires in the actual Star Wars saga, it becomes apparent that there is an answer to whether lightsabers can survive even in the most extreme weather conditions. Even more remarkable, real-world physics supports this concept.
Lightsabers Underwater
In Star Wars lore, there are multiple occurrences where lightsabers are wielded in space and underwater. However, many of them require proper material in order to ensure they can still function in these locations.
Kit Fisto memorably wields lightsabers underwater on Mon Cala during the now non-canon Star Wars: Clone Wars cartoon. However, several Jedi, from Ahsoka Tano to Luke Skywalker, found their lightsabers were waterproof. This is due to a lightsaber modification that, at least in Legends Canon, Kit Fisto himself invented. It's called the Bifurcating cyclical-ignition pulse. This augmentation uses two crystals to create a waterproof casing to protect the lightsaber from water. Otherwise, the circuits would short-circuit, causing the blade to dissipate.
Because Kit Fisto's homeworld, Glee Anselm, is primarily a water-based planet, it makes sense that Kit Fisto made such an augmentation to his lightsaber. Several other planets are also mainly aquatic, which makes it necessary to have weapons that can function underwater. In the 2020 Darth Vader comic series, the Sith Lord himself had to draw his lightsaber underwater on Naboo.
The Bifurcating cyclical-ignition pulse first made its appearance in Legends canon long before Kit Fisto first appeared in the book Young Jedi Knights: Crisis at Crystal Reef. In this case, we see that Luke's new generation of Jedi all design their lightsabers with the Bifurcating cyclical-ignition pulse already installed, as opposed to prior generations of Jedi who included it voluntarily. Without this design, the lightsaber would short-circut.
As for the plasma beam itself, it is possible that the water might cool the plasma. However, due to it being super-hot matter, it is possible that the water around the lightsaber might start to shift forms as well. If plasma touches water, it causes water around it to boil. At roughly 1000 degrees Celsius, that water becomes plasma itself. In the aforementioned Star Wars: Clone Wars scene, Kit Fisto's lightsaber causes the water around it to boil.
Lightsabers in Space
Lightsabers in space are seen throughout canon, most memorably with Plo Koon in the second episode of Star Wars: The Clone Wars. Lightsabers contain the plasma generated by their fuel tank and kyber crystal through its emitter matrix. So long as the components aren't damaged, the blade functions normally wherever. Therefore, given that the plasma can be generated, it will function. There is no risk of short-circuiting in space as there would be underwater without augmentation.
Plasma can materialize anywhere as long as it is hot enough, so it could exist in the vacuum of space. The sun is evidence of this; it's a ball of plasma, one that self-sustains its existence in space. Ergo, there is reason to believe that a beam of concentrated plasma — a lightsaber — could function in space.
Lightsabers can exist as weapons wielded in the vacuum of space or at the depths of the ocean. The only issue appears to be the physical qualities of the hilt and mechanics itself, not if the blade will snuff itself out magically. It's safe to assume that no environment is safe from the Jedi's weapon of choice.