Or you can use underwater speaker and hydromicrophone
https://www.google.ca/search?rlz=1C1NCHB_en&biw=1440&bih=787&tbm=isch&sa=1&ei=mT73Wti0POeGjwS7kbqIAg&q=arduino+hydromicrophone&oq=arduino+hydromicrophone&gs_l=img.3...11755.18264.0.19116.10.9.1.0.0.0.227.745.8j0j1.9.0....0...1c.1.64.img..0.0.0....0.pV2fBaEt5fI