Which filament to use for an application involving water circulation? Let's suppose I want to build a water block shaped exactly for my Ubis 13s hot end 



(similar to this instructable), or that I want to circulate water around specific objects to be cooled (like Water-cooling stepper motor with aluminum block).

CNC allows the use of shaped radiators/water blocks, but it is too expensive for home projects and requires the ability to make a part at the first or second attempt, to keep costs low.

3D printing allows me to design jackets which are perfectly matched to the intended part, including gaps for O-rings or matching valleys+ridges between the components which also would result in a mostly waterproof seal (just add grease or hot glue...). In this case, water would be in contact with the object to cool and also provide (where suitable) a better interface. Corrosion can be avoided with some additives (in case of aluminium, water should be acidic).

Which common printing material is best for this application? Let's limit the question to materials that can be reliably printed at maximum 270&nbsp;°C (nylon ok, but no polycarbonate or PEEK, for example).

We are talking about very low pressure pumps, mechanical strength is of little relevance.
I saw a lot of selling PETG water cooling pipes.
So PETG will be the best choice considering all factors.

In addition:
You could use any plastic with the oil instead of water for cooling medium.
Also automotive anti-freeze (G11, G12, G13 ...) designed to be non corrosive and much better than water in terms of cooling.