Let's suppose I want to build a water block shaped exactly for my Ubis 13s hot end
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 °C (nylon ok, but no polycarbonate or PEEK, for example).
We are talking about very low pressure pumps, mechanical strength is of little relevance.