forgotten title


Years ago a read a great science fiction book but I can't remember the title or author. I wonder if any of you recognise this synopsis?
Humans need to be physically altered to pilot space craft - most people go into long-term stasis when travelling through space but the pilots undergo several operations to incease their heart functions etc.
Back on earth, people can choose other modifications, including gills. The hero of the book seems to be able to do most of these things without modification and he spends time communing with Blue whales.

ring any bells?
The first sentence had me thinking of several different anime works (eg: the military mech pilots from Eureka 7) but the second sentence describing the gills caused me to come up blank.

Sorry, no thoughts. :(