Well, you need it to work better than without the magic sugar water.

My approach is: I believe that the strategy of "if the magic sugar water worked with only 1 in a million probability of 'worked' being obtained by chance without any sugar water, and if only a small number of alternative cures were also tried, then adopt the belief that magic sugar water works" is a strategy that has only small risk of trusting in a non-working cure, but is very robust against unknown unknowns. It works even if you are living in a simulator where the beings-above mess with the internals doing all sorts of weird stuff that shouldn't happen and for which you might be tempted to set very low prior.

Meanwhile the strategy of "make up a very low prior, then update it in vaguely Bayesian manner" has past history of screwing up big time leading up to significant preventable death, e.g. when antiseptic practices invented by this guy were rejected on the grounds of 'sounds implausible', and has pretty much no robustness against unknown unknowns, and as such is grossly irrational (in the conventional sense of 'rational') even though in the magic water example it sounds like awesomely good idea.