Not to mention that there are five star systems closer to Sirius than our own is, including Epsilon Eridani, which has a much denser debris disk than Sol and would thus probably have considerably more water ice. The total amount of water contained in such a disk would be millions of times more than a single inhabited Earthlike planet could possibly need.
I discount Sirius itself because it could in fact be a very arid, waterless system due to the great heat of the A star and the fact that the B star has already gone through its death throes, which probably seared the system of any volatiles. Which of course renders it pretty much impossible that it could have a native intelligent species, but there's an off chance that the Visitors aren't native to the system but terraformed one of its planets for some reason, bringing in their own water to hydrate it. Perhaps if they're still in the process of terraforming, they'd need a source of extra water; but there would still be far more convenient and practical sources for it than Earth's oceans.