The cause of watery stuffed shells is usually using watery, or runny, ricotta. In general, whole milk ricotta tends to be less watery. Regardless, i your ricotta seems on the watery side, spread it out onto a clean kitchen towel and press it with a second towel to absorb some of the excess moisture. Alternatively, scoop the ricotta into a cheesecloth lined fine mesh strainer and let it sit for an hour or so. The excess liquid will drain from the ricotta as it sits.