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There are two likely causes. First a loose hose clamp on one of the cooling hoses has come loose and caused the cooling system to become inoperative. This occurs occasionally. Reference: http://www.pwctoday.com/showthread.php?t=191804
It will require a check of all the cooling system hose clamps to determine if this is the cause.
The second and most likely cause is sand or seaweed has clogged the cooling system passages. This will require an inspection of the jet pump cooling water inlet, clearing any debris and flush of the cooling system to repair.
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