Most hot water cylinders are still made from copper, they just have a fiberglass jacket around them to hold the insulation in place. The open top tanks found in lofts are for central heating systems and not directly connected to the hot water system.
Oh, thank goodness. I couldn't imagine having that disgusting water from a tank with rotting animals in it touching me! Eww!
On tv we have a programme called cowboy builders - conmen builders - and this plumber got caught on hidden camera for peeing into the loft water tank of the house that he was fixing. Yes he was fined etc Disgusting isn't it?!