Hi to all, i´m new here!
I dont have expierience on D2-55, but with elder Mercedes OM616 engines, which are quite the same stuff.
You said your problem is on the first cylinder (the opposite side to the saildrive!). If this is correct, it seems unlikely to be raw water from the exhaust System (the water has to pass all other cylinders to proceed to the first one!). More likely seems to be a leak in the head or the head gasket. If you have a leak in the closed inner water circuit, there is not neccessary a noticeable loss of cooling fluid. In such cases sometimes the inner coolant circuit gets under pressure from the combustion gas and causes change of colour of the cooling fluid. You should check this
Excuse my bad english, but i hope it helps.
Robert