If you are a rejistani, there is a high chance that you are an inikresaist, ie: an adherent of the inikresaist religion. In this religion, rivers and oceans as well as the jungle and even cities are inhabited by spirits and these spirits can interfere with the life of people. If rivers, lakes or seas get angry, it can cost the life and ship of whoever was on it at that time. Thus calm water means that the spirits agree with the fact that you want to cross it. It can of course also just be used in the very mundane sense of agreeing with someone (xe’hjani Nik: I agree with Nick).
Example: Jui’het mi’hjani’ta xumaj’het’jet. (Ocean 3S-be_calm-NEG storm-TEMP: The ocian is not calm during a storm)