Engine Water Leak

There are a few different solutions here in the forum in several different threads. Try searching for “hot water heater” or “fixture leaking.” Just make sure to select advanced search settings and change from the default time frame of “last 6 months” to “all.” The easiest solution is to leave the water pressure pump off when not in use - especially while motoring, and leaving a hot water tap open while motoring or heating water at the dock. This keeps the tank from overpressurizing. You’ll find other methods involving drain tubes/tanks and accumulator tanks.

Congratulations on your new boat!

On my 341 and I am guessing your 371 will be very similar, the seawater is picked up through the saildrive, leg, through a strainer, then to the belt driven sea water pump into the heat exchanger then through an anti-syphon loop with a anti-syphon air vent valve, (above the engine), then into the exhaust system.   The anti syphon air vent will probably have a drain valve that may drip a bit when the engine runs, so try and ensure if drips into the area under the engine.  

Normally this system has sea water in it, but at the end of the season, I will shut the saildrive valve, and put a pipe onto the sea water pump and pump anti-freeze through the sea water system.  Probably a bit of an overkill for the West Coast of Scotland, but better safe than sorry.  With the boat being new to you, is this something the previous owner may have done which may be confusing you with the colour of the water?  Apart from the obvious pipe connections, other sources of leaks are the seawater pump cover and the seawater shaft seal. (Mine leaked last year.)  First approach could be to put tissues in certain areas so you can see where the first drops appear,  The pump will have a slight pressure head on it from the seawater strainer, so you could try shutting the saildrive leg valve, draining the pipe feeding the seawater pump and see if you still get leaks.

The engine cooling system has a pump in the engine which will pump the cooling water back to the the calorifier and then through the other side of the heat exchanger.  Somewhere there will be a branch pipe off to the expansion header tank. 

Mike is dead right on the issues with the calorifier pressure reducing valve, but I think that will just drain into the main bilge, whilst I am surmising that you are talking about the area just underneath the engine.

The only place the engine systems interface (but not connected) with the domestic water systems is in the calorifier with them being on different sides of the heat exchange system

I have drawn some very rough sketches to show the piping systems on my 341 and I think your yacht will be very similar, so pm me your email address if you are interested in them.