If you can get a 12" drop from the exhaust outlet center line to the input of your water lift
muffler you are usually in good shape. The attached page from the Volvo Penta
manual shows drops considerably less than that.
Considering today's flat bottomed boats, in some cases its very difficult to find the amount of drop needed, and an exhaust riser becomes necessary.
The other thing to consider here is that the muffler shown is the VP rubber hose type, with the inlet near the top of the canister, and the outlet picking up the water close to the bottom. This makes the effective height from the elbow to the internal static water level greater than it seems at first glance.