hot water is dirty?

Without any information on what kind of boiler or system you have its impossible to say why.
However if its a cylinder I would be suspecting either failure of the coil on an indirect cylinder ie central heating water mixing with your bathing water or failure of the bubble on a primatic type (this would usually happen if you have had work done on the system).
Either way I would get it checked out asap as you may not fancy bathing in Fernox.
I am guessing its not a combi.

Andy the gas man
March 2006