Another possibility is [**The Songs of Distant Earth**][1] (1986) by Arthur C. Clarke. From Wikipedia:

> The novel is set in the early 3800s CE, and takes place almost
> entirely on the faraway oceanic planet of Thalassa. Thalassa has a
> small human population sent there by way of an embryonic seed pod, one
> of many sent out from Earth in an attempt to continue the human race
> before the Earth was destroyed.
> 
> The story begins with an introduction to the native Thalassans – the
> marine biologist Brant, his partner Mirissa and her brother Kumar.
> They are typical examples of the Thalassan culture; quiet, stable and
> free from religion and supernatural influence. Their peaceful
> existence comes to an end with the arrival of the Magellan, an
> interstellar spaceship from Earth containing one million colonists who
> have been put into cryonic suspension.
> 
> In a series of descriptive passages, the events leading up to the race
> to save the human species are explained.

I do seem to remember some humans genetically modified to make use of the aquatic environment but didn't seen anything to that effect in the plot description.


  [1]: https://en.wikipedia.org/wiki/The_Songs_of_Distant_Earth