Thank you both for providing your answers. Unfortunately, none of the translations is 100% correct: Gunner's Knot (as shown here:
http://www.gutenberg.org/files/13510/13510-h/13510-h.htm, fig. 37) has one final knot - which makes it "impossible to undo" and also different from the knots suggested here in Czech.
Prior to posting my question I had already been familiar with both "hadí" and "svírací" (uzel) and "konstriktor"; I was hoping to get a more precise answer/translation here.
Having consulted a sea captain I got to know that not only is this particular knot hardly ever used at sea (!) but also has no proper Czech translation. I was adviced to translate indirectly as "lodní smyčka s pojistkou" which I am going to do.
However, thank you once again for your time and effort!