Firstly, water. I was told never to run water (unless distilled, even though, just don't do it).
Water expands in heat and boils = added pressure on the radiator = fracture. Whether it be a hairline or a large crack, when things heat up, the crack will open up and thus goes your coolant. Water can also cause rust and I've seen some pretty good rust f**k on engines where they've completely disintegrated the inlets.
Secondly, find out roughly what area it's coming from, either front, middle or back of engine bay.
Front - Radiator
Middle - Possibly some hose loose or cracked due to age or other chemicals that could have come in contact and dissolve a part. Highly unlikely that it would be anything serious like engine parts, but it is still a factor to consider.
Rear of engine bay or firewall - Likewise with middle, but could also be the coolant running into your firewall.
I wreck mainly R33 and R34s and with them, yes there an occasional problem but since your car is a V35, I'd guess there would be better stuff in there.
My suggestion:
Diagnose where the coolant is coming from, change the radiator if that is the case.
Pick up sludge or rust remover additive at your local auto store and run it according to the instructions on the bottle (usually has to be left in for a certain ammount of KMs)
Buy Anti-Freeze & Anti-Boil Coolant
Once leak is diagnosed and repaired, additive is added into coolant and run for the specified KMs by manufacturer of the additive, flush and refill coolant with the same stuff you used before.
Bit of work but would save you a heap in the long run, also gives you the peace of mind that it won't happen again, not in your ownership anyway. Also don't run your car when the temp is right up to H. I don't run my 34 higher than 75% of the temp gauge.
Estimated Total cost if DIY: $250
Edit: Don't forget to bleed the air out as well. There should be a screw plug thing within the coolant line that is the highest point of where the coolant runs, unscrew it, run your engine till it sits at its normal temp, get a mate to help or pull on the throttle cable in the engine bay (MAKE SURE IT IS IN EITHER NEUTRAL OR PARK AND HANDBRAKE IS ON, otherwise you will run yourself over with a phantom driver). Leave it off until you get a constant stream of coolant running out. It will spit air, bubbles and coolant out then when all air is bled, constant coolant. Bleeding will require you to replace the coolant lost as it goes. I do this by using a 1.5L coke bottle, cut off at the bottom, no cap and tipped upside down to where the radiator cap is, hold it down firm and funnel coolant in. You'll see the level rise up and down and up again and down lower again as it flows through the radiator.
This is a general idea coming from R33/R34 RB25 experience.