Toss cucumbers and onion with salt in colander set over large bowl. Weight the cucumbers down to press out as much moisture as possible using a gallon-size ziploc bag. Allow them to drain for 1-3 hours. Rinse and pat dry.
Bring 2/3 C water and 1/2 C vinegar to boil in small saucepan over medium heat. Stir in sugar to dissolve, then reduce heat and simmer for 15 minutes. Let cool to room temperature.
Mix the cucumbers, onions, and jalapenos together then pour the dressing over the top, tossing to coat. Chill until ready to serve.