Some boat motors have a bypass thermostat system where the raw cooling water bypasses the engine until the engine heats up, then the thermostat closes and forces the water thru the engine. If your system is like that, the thermostat might be not closing enough and at idle too much of the water is bypassing the engine and going right back out to sea. The increased flow at higher rpm's could be forcing adequate flow thru the block.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13