Your first picture is of the wrong line.... that is the water outlet from the exhaust manifold and intercooler.
The goal of the tow valve is to prevent water from flowing through the exhaust manifold into the water box. If the water box fills with water while the engine is not running, the water will go up the exhaust and into the engine.
When you tow above a certain speed, you generate enough water pressure at the pump to force water through the exhaust manifold and into the water box. Seadoo recommends clamping line "A" if you are going to tow (do not clamp line A1 or A2). Clamping or placing a valve in this line (position V1) ensures no water will flow into the water box.
Other people have decided to put a valve on the line leading from the jet pump to the exhaust manifold inlet. They either put it in position V2 before the intercooler or V3 after the intercooler tee. This prevents pressured water from the jet pump from flowing into the front inlet port of the exhaust manifold from the jet pump in the first place. With this approach you may still have some risk of getting water into the water box. It may be unlikely, but I would think you could still get some water to flow backwards into the rear outlet of the exhaust manifold from the outlet of the intercooler and into the water box.
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