Issur to Eat
30. Once it gets dark, one may not eat anything or even drink wine or other beverages [unless he is still eating seudoh shlishis] until he says Havdoloh with a cup of wine (שו"ע סי' רצ"ט ס"א). The reason for this is not because he may end up forgetting to say Havdoloh [which is the reason for the issur to eat before doing certain other mitzvos]; it is an inherent issur out of respect for the mitzvoh of Havdoloh, like the issur to eat before Kiddush (שער הציון סק"ז).
31. Saying “Boruch Hamavdil.” Saying “Boruch Hamavdil…” only allows one to do melochoh (below, 35); it does not allow one to eat. Therefore, even if one said “Boruch Hamavdil,” he may not eat anything until he says Havdoloh with a cup of wine (מ"ב סי' רצ"ט סק"ט).
32. Women. Even though women should not say Havdoloh for themselves (above, 23), they should still be careful not to eat anything before hearing Havdoloh (שו"ת אמרי דוד סי' כ"ב).
Water
33. One may drink water before Havdoloh since water is not considered a significant drink (שו"ע שם). Still, based on the Arizal, one should not even drink water before Havdoloh (כף החיים סק"ו, ליקוטי מהרי"ח).
34. Seltzer. One should not drink seltzer/soda-water before Havdoloh since it has a more significant status than water, as it has somewhat of a flavor and even someone who is not thirsty makes a brochoh on it (מו"ר בשו"ת שבט הקהתי ח"ב סי' קכ"ד).
