Finding water is exciting mostly because on Earth, where there is water, you find life (generally). I haven't followed astrobiology of recent years, but I remember about 8 years ago people were looking for water on Mars as evidence that life might have existed there, or may still exist there- and now, of course, the next step is to look for evidence of life. If they find things sufficiently similar to Earthly organisms (say, bacteria containing DNA), it would be hugely significant- this would support the theory of panspermia, which is the idea that life on Earth arose originally from bacterial cells present on comets that collided with the Earth.