RTD's Water Theme
aka The Big Watery Agenda
Draining the Thames in The Runaway Bride
The throwing of the ruby in to a lake to stop the Vespaform in Unicorn and the Wasp
Donna staying behind by the pool in Midnight
The Doctor drowning in Turn Left
In Stolen Earth the Albino servant tells Donna: "You need sustenance. Take the water. It purifies."
The Cyber King emerging from the Thames in The Next Doctor
The Medusa Cascade
The Waters of Mars
There's probably more but I'd have to rewatch all every episode.
Is it leading to something, like the return of the Sea Devils in Season 5, or just a series of coincidences?