Remaining on the idea that cells and viruses hold interesting physical characteristics that hold the ideas of alienation and aquatic life I thought it would be interesting to see what is out there in terms of this interesting combination.
One example of an aquatic alien would be these shown in the Doctor Who special 'The Waters of Mars', in which the crew of a mars base become aquatic alien nightmares after exposure to what is said to be a virus frozen in an underground glacier on Mars. The infection is extremely quick and can be transmitted just by touching infected water (one drop is all it takes). The result ends up with the victim looking like this guy:
Perhaps a more recent example of an alien virus is the so called 'Manhattan Virus' seen in Crysis 2.
Created by the alien race the 'Ceph', a supposed aquatic species due to their gelatinous form, this virus is used for molecular breakdown, similar to the way HIV disables the immune system. The virus plays with this idea of aquatic origins in that its shape, and movement for that matter, are strikingly similar to that of a squid or octopus - it swims in its victims body fluids. In terms of colour it has an X-ray quality to it which is likely used to show how it functions and spreads.