boat.onclick = function() {
  //...
  let times = 1;

  function go() {
    if (times % 2) {
      // navegar a la derecha
      boat.classList.remove('back');
      boat.style.marginLeft = 100 * times + 200 + 'px';
    } else {
      // navegar a la izquierda
      boat.classList.add('back');
      boat.style.marginLeft = 100 * times - 200 + 'px';
    }

  }

  go();

  boat.addEventListener('transitionend', function() {
    times++;
    go();
  });
};