Yelling doesn’t carry as far as a whistle, requires vast amounts of energy, and damages the voice and throat with extended attempts. So use your whistle. A universally-recognised distress code is three equal blasts on the whistle, to be repeated until others hear it and respond with two blasts of recognition. You can also use the universal SOS distress code (three short blasts, three long blasts, three short blasts), but know that three equal blasts in a row is sufficient and indicates a distress situation.