Here you go,
stick a torch straight on the top of the tank (works best in the dark) and you can clearly see the coolant level as the torchlight makes it glow. i've juat taken the pic below so you can see - my coolant is right where it should be, just about halfway between the 'full' and 'low' lines on the rear of the tank.



the level obviously is best kept between the marks, however dont lose too much sleep if you have to run it a short distance before you can top it up. the coolant tank is merely an expansion tank rather than a header tank.
An expansion tank receives the excess coolant that is expelled from the radiator when the system is hot, whereas a header tank is park of the pressurized system.
What this means for the Tenere/X/R is that providing there is at least SOME coolant in the tank when the engine is COLD then you wont cause any problems by running it for shorter trips, but ensure the tank is topped up to the correct level as soon as possible.

If its merely a top-up (i.e. less than a cupful) then tapwater is fine, it wont dilute the mixture enough to cause problems.

(i accept no responsibility for anyone being an ass and taking this advice too far and running their bike with no coolant in it. if my advice is too complicated for you read your owners manual. if thats too difficult go to a yamaha dealer!)

P.S. Ryben - stunning part of the world you live in, one of my favorite areas - you have some truly cracking roads for rides