A Torrent of Faces — James Blish

The time is the end of the 28th century. A world government is attempting to deal with the problems created by a population approaching one TRILION souls. Living space is at a premium; every possible square foot of arable land is in use; and there still is not enough food.

The only hope for ultimate survival lies in the long-awaited discovery of an interstellar drive. And just as every resource is strained to the breaking point, a new race, the Tritons, emerges from the oceans. They mingle with the humans and they demand their rights as Earthmen!

The theme of dedicated humans vs impossible odds runs all the way through this long novel. It's a good one, sadly lost to print.