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The water pump is an easy check and fix....what about a head passage that is clogged...you mentioned you replaced the riser and manifold but the temp sensor is ususally screwed into the head...the temp you are measuring is either the engine block or head. If its not the pump it sounds to me like you have a restriction internal to the head which the pump can't push enough water thru at high speed. to keep her cool. The therms are probably wide open.
Salt water cooling is a nightmare I didn't know they still make that on inboards. I had a SWC "grey marine" 225 inboard once....what a nightmare. Repowered with a crusader 350ci fwc and ran for many years without problems.
240 is pretty damn hot (fw water boils at 220), you probably saw some serious steam out of the exhaust.
If the pump checks out you may have to have the heads boiled out.
good luck.
(don't forget ...short blocks are cheap)
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