The new swiper Emacs package is proving to be a great alternative to helm-swoop. Here's how to prefill with current region:
(defun ar/prefilled-swiper ()
"Pre-fill swiper input with region."
(interactive)
(if (region-active-p)
(let ((region-text (buffer-substring (region-beginning)
(region-end))))
(swiper region-text))
(swiper)))
(global-set-key (kbd "C-s")
#'ar/prefilled-swiper)