On the 787 end of life subject, I doubt very much that they'll end up as an artifical reef or scuba diver's plaything once they come to the end of their life for a couple of reasons.
Firstly the surface of fully cured, "as molded" composites tend to have a very teflon-like quality in that it is very difficult for something to attach itself to the surface, such as plankton or other coral forming organism, unless the surface has been abraded to promote adhesion.
Secondly, the resins used in the composite manufacture are fairly to extremely toxic in their unreacted form. They tend to have relatively low vapour pressure, but can be resily soluble in water, so the chance that any might get into the marine environment over the long term is probably higher than might get into the passenger environment over their working life. Also, composites can be permeable to water, allowing unreacted monomer to be washed out of the solid structure over the long term.
Looking at what happened to an RAF Harrier a couple of years ago which crashed in the UK, a signiifcant amount of composite materials were damaged in the crash & contaminated a large area. I understand that the affected area was scraped clean of topsoil to a depth of a couple of inches and this resulting material was thereafter incinerated at high temperature to remove all traces of carbon fibre and resin particles. The remaining material was then analysed and declared safe once no composite material was found. I understand from a freind who worked for BAE on composite structures at the time that the incinerated material was eventually mixed with organic matter & finally used to fill a flowerbed at the RAF base used to recover the wreckage.