Bruce - Did you use your IR temp gun on the riser/elbows?
Since the elevated temp you see on the gauge is apparently underway & probably at cruise RPM, I believe the likelihood a scored RW pump cam or housing is the issue is not great, as that is more likely to cause a low suction/overheat near idle RPM. At higher RPM, the impeller blades supposedly don't even contact most of the housing and it acts like a centrifugal pump, with help from the hull pickup scoop underway.
Noticeably excess steam is usually either caused by a head gasket leak , manifold or riser/elbow water passage or gasket leak, weak RW pump, RW flow restriction before or after the pump or suction air-entrainment . If it was head gasket or manifold leak on your RWC engine you'd smell distinctive A/F odor in the exhaust. Suction air leak would likely also cause temp rise at low RPM, but it's worth checking for anyway( hoses , clamps , seat strainer lid o-ring, etc. .
Underway, put your hand on the top of the riser/elbows of both engines.. Can you keep your hand essentially indefinitely on the ones on the "cooler" engine but it's quickly painfully hot on the warmer one? If so that's a pretty good confirmation the hotter engine has a RW low-flow issue to cause that extra steam.
You already changed the impeller, and riser/elbows, "all"(?) RW hoses, but you didn't mention back flushing or rod/inspecting all coolers and the H/ex which can definitely get restricted in spite of upstream strainers.
Smitty mentions the wire wrapped RW hose. Sometimes the thin-wall RW pump suction side hose with inner support wire coil will have the coil corrode and disintegrate ( 2 or 3 times on mine so far), allowing the suction hose to collapse to some degree at higher RPM pump demand. But you said "all new hoses." Similarly the large hose from the heat exchanger to the circ pump may also have a wire coil that can migrate with flow away from the h/ex outlet just enough to allow suction to collapse and restrict flow enough to raise temps at higher rpm but that is on the FWC side and would not affect riser temps or cause steam . Plus "all new hoses" .
Raw water flow spec for at least the Crusader 5.7L engines is MINIMUM 5 gallons total in 15 seconds at 4000 RPM. The 7.4 L engines may have a greater min flow spec.. That's a lot of flow, so you can have maybe 1/2 that and it is still going to look like a lot of water in the exhaust- but it may not be enough to sufficiently cool exhaust temps below that at which some of the RW turns to excess steam.
So the 1st thing I would check as Jamie suggests is the RW scoop strainer. Growth fouling may well be the culprit but even A/F paint buildup that was not raked out prior to splash is often a cause for substantial RW flow restriction. (A year ago I changed all my scoop strainers to Groco's with hinged hatches for easy cleanout and ability to apply A/F on the inside passages. but I still rake the pickup slots before commissioning as the yard always slaps A/F on thick and doesn't followup with re-opening the slots. )
Make sure there are no overly-tight bend/kinks in any of the new RW hoses.
Then I would rig up dock hose adapters and back-flush oil and transmission coolers into a bucket so you can see results which may surprise you.(I do this at least once a year and find sea lettuce that is a lot bigger than could have made it through the scoop strainers or the sea strainer baskets.)
Then check the RW side of the h/exchanger. If the exhaust hoses are in the way of the end caps (as on mine), you might be able to just back-flush there too, but hopefully those were opened & checked when the exhaust hoses were disconnected from the old risers/elbows.
Best luck.
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