Somewhere it will also tell you to keep the blanket dry! Electricity and water don't mix I'm told.
Water and electric don't need to mix, you are covering your tank etc not putting it in it! A lot of the modern electric blankets are now washable in the washing machine anyway. In the the extremely unlikely 100 million to 1 chance there was a fault then it would cut out or just plug in to a safety cutout socket or whatever they are called, the same people will do with their electric fan heaters etc. Just a tip I thought I'd pass on but if anyone feels its unsafe then please ignore it.