To understand why you need a lift to accomplish this task, try doing this little exercise. Seriously, try it to see what I mean. Lay on your back and put both hands over your head, up about a foot or so, as if you were working on the vehicle. The water pump pulley is on your right and the crank pulley is pretty much directly overhead. Now take you right hand and raise it with your fingers extended as if you were pushing up on the water pump belt and its pulley with plenty of force to keep the belt in the groove. Bear in mind that you are exerting this force while the back of your hand is facing the engine, not your palm, and you are trying to push it toward the engine. Now simultaneously take your left hand and pretend you are pulling down on the ratchet attached to the crank pulley right over your face to rotate the pulley. Bear in mind here that if the ratchet slips as you pull down against the engine compression it lands squarely on your face (ouch! Ask me how I know.). The contortions of this approach are horrible to say the least. You might be able to do it if you positioned your body completely under the vehicle so that your head and shoulders are the only thing in the wheel well and the hand functions were reversed, but I wasn't going to try that even with jack stands (call me a coward).
Now imagine you that have the vehicle up on a lift at the right height right in front of you. For this exercise, stand straight up with your hands comfortably over your head, maybe 2 feet or so this time so that your elbows are straighter than in the other exercise. Imagine your right hand is pulling down on the crank ratchet and your left hand, palm facing the engine, is now guiding the belt onto the pulley where you can use your fingers and your thumb to help. Much, much better body mechanics, no? You can easily see how you can apply much more force with both hands in this situation and that "stretch" water pump belt needs a lot of force. With a lift it's even possible to have a helper in there with you. So even though the clearance is exactly the same in both situations, your ability to apply the mechanical force needed is vastly different between the 2 approaches.
Your plan to only replace the tensioner and the belt is a good one. Wait for the other belt to get totally worn out and then find a lift (or a mechanic)!