Might have to do with density of the water which is influenced both by temperature and depth.<p>Something similar happens with sound but the range is way off.<p><a href="https:&#x2F;&#x2F;en.m.wikipedia.org&#x2F;wiki&#x2F;SOFAR_channel" rel="nofollow">https:&#x2F;&#x2F;en.m.wikipedia.org&#x2F;wiki&#x2F;SOFAR_channel</a>