This is a long shot, but the book is possibly [Jack the Bodiless][1] by Julian May. I think it's a long shot because _Jack the Bodiless_ has an involved plot that is a lot more complicated than your question suggests. However it does match your description in many ways.

[![Jack the Bodiless][2]][2]

>In the year 2051, Earth stood on the brink of acceptance as full member of the Galactic Milieu, a confederation of worlds spread across the galaxy. Leading humanity was the powerful Remillard family, but somebody--or something--known only as "Fury" wanted them out of the way.
Only Rogi Remillard, the chosen tool of the most powerful alien being in the Milieu, and his nephew Marc, the greatest metapsychic yet born on Earth, knew about Fury. But even they were powerless to stop it when it began to kill off Remillards and other metapsychic operants--and all the suspects were Remillards themselves.
>
>Meanwhile, a Remillard son was born, a boy who could represent the future of all humanity. His incredible mind was more powerful even than his brother Marc's--but he was destined to be destroyed by his own DNA...unless Fury got to him first!

The eponymous _Jack_ is the "Remillard son", _Marc_ is the brother and the aliens with the mating pools are the _Krondaku_. The scene with the mating ritual is:

>Loga'etoo opened the door to the viewing chamber arid gestured with a tentacle for the children to go in. They found themselves in a black-rock grotto, almost entirely filled by a dark pool. The vibes of the place were strange-scary and thrilling at the same time-and the air temperature was no longer chilly but pleasantly warm. All of the illumination came from deep within the pool, where indistinct large shapes that glowed with shifting, throbbing colors were languidly adrift. Racing swiftly and erratically among the great shining masses were a few smaller ones.
>
>Loga'etoo's mind spoke: We will move to a place where we will be able to survey the scene beneath the surface. Please do not speak. Many Krondaku hold their sexual congress to be sacred, as do certain humans.
>
>Marc trailed after the others as they descended a narrow ramp. At the bottom was a great transparent window simi lar to that found in some Earthside aquariums. Now it was possible to see more clearly the Krondak couples, conjoined at the ventral surfaces and suspended in the dense liquid. Their huge bodies, so shapeless and hideous on dry land, had a strange rippling grace when afloat. The tentacles of the mated exotics curled and uncurled in rhythmic mutual motion, and what had once been mere ugly warts on the blotchy Krondak integument were transformed into multi colored luminescent organs pulsating in slow synchrony to the sexual tempo.


  [1]: https://en.wikipedia.org/wiki/Galactic_Milieu_Series#Jack_the_Bodiless
  [2]: https://i.sstatic.net/gYwTe.jpg