p.17 of the Guide (3rd column, 1st para) confirms that Glorantha has "Salt seas", and p.460 tells us that "seas are large saltwater bodies".
p.460 also tells us how everywhere was water at first, then the surface world was pushed up from the depths, and rivers crept across the land. Was the initial water salty and the rivers somehow lost their salt, or was the original water non-salty and somehow became salty later? Or something else? The former seems more likely to me, but we obviously need a mythic explanation for this.