Skip the water (use an empty, foiled pan) and you should not have trouble achieving that temp.
Cooking the roast low will mean that you should have very even doneness throughout. (As opposed to cooking at moderate (325) or high temps, where the interior of the ends and sides will be more done, the center less.)
Leave the roast out of the fridge for 2 hours before cooking to bring it closer to room temp. You're probably looking at ~4-5 hours cook time, but actual cooktemps will determine this. (If it is going too slowly at the 3.5-4-hour mark, bump up the temps a bit; if quicker, it will hold.)
You can cook the roast start-to-finish at your target. For a more caramelized, flavorful exterior, either sear it first (in a grill or in an oiled pan on the stove) before moving it to the WSM, or cook it in the WSM from the beginning, then, when 7-10? shy of your target internal finish temp (~125 should be your target if your preference is rare/med-rare, so you'd look for 115-118?), do a finishing sear on another grill, or on the WSM (by dropping the grate with the meat to the coal ring and searing it there, rotating it frequently, till nicely colored), or on the stove, in an oiled pan. Take it no higher than your target once searing begins. Rest the meat, tented, 15-20 minutes, before carving.