In general, the Beracha of Hamosi covers all foods eaten in a meal. However, it does not cover wine, which has a special status as "Chashuv" (important), since it is used in many Misvot ceremonies. Therefore, if someone drinks wine during the meal, he must recite a separate Beracha of Hagefen.
The question is whether other beverages, such as water, are covered by the Hamosi. The Shulhan Aruch (Siman 174) brings two opinions on this matter. The first ruling is that other beverages are included, because they are an integral part of the meal, as opposed to wine that has a unique status. The second opinion holds that Hamosi does NOT cover water drunk in the meal. The Halacha is in accordance with the first opinion.
Nevertheless, the Shulhan Aruch recommends to be strict and avoid this situation altogether by reciting the Beracha of Shehakol on the drinks before he washes, so as not to have to decide between the two opinions. One can also recite the Shehakol on a food item, e.g. a candy, as well, and intend to cover the beverages in the meal.
Hacham Ovadia suggests another way to avoid the debacle by drinking wine in the meal. The Hagefen recited on the wine will cover all other beverages.
The Mishna Berura (Rav Yisrael Meir Kagan of Radin, 1839-1933) offers another possibility. During the meal, one can listen to the Shehakol recited by someone else, in order to cover the beverages he wants to drink.
The Beracha of Hamosi covers all the beverages to be drunk during the meal, except for wine. Nevertheless, it is praiseworthy to avoid a situation in which one relies on the Hamosi as the Beracha on other beverages.