I Agree with TB. Asked the same question a few weeks ago, and looking at the threads doesn't seem to explain it to me either. What you describe seems to be the norm, but I now have a Sleekcraft where the numbnuts who put the new engine in, in changing from OE TT's to Rewarder OT's, ran no overboard dump on anything. Obviously explaining why the brand new engine only ran 20 minutes. It's a little more compicated than that, and I need to post pics of what seem to me to be the most unusual T-stat housing and Water pump cover I've ever seen on a jet boat.
Long and short of it, I beleive I need to run a releif valve above all else. Still got to figure the overboard dump thing, but I do own drills that can do this, lol. Still haven't figured out if I need to run a thermostat or not. It all seems basic until you really get into it on these things.
Tom from JBP will chime in eventually, hopefully, with the DEFINATIVE answer.
Ray