Good news
No, you don't have to remove the exhaust riser from the manifold.
What you need to do is buy what is called a "hump hose" available from Catalina Yachts.
The blue hose is the HH. It's a special 1-5/8 inch size that Trident makes only for Catalina Yachts. My exhaust riser split at the raw water intake nozzle. Check your riser while you're there.
Pictured is the old stiff hose that I formerly used before I bought the hump hose. It tends to crack the muffler intake port. You could use it if you chose, but since the HH is available, why not use it, 'cuz it's better.
Just disconnect the old hose from the end of the exhaust riser and the muffler and install the hh.