Yes, there is.
All chargers are not created equal. If you are purchasing your chargers from third party marketplaces, like Amazon, you can’t be sure that your product isn’t counterfeit. 90% of iPhone chargers listed on Amazon are counterfeit, according to Apple. In a study conducted by UL, 99% of those 90% failed basic safety tests, and 22/400 had the capacity to cause electrocution. Unless you open it up, it’s impossible to know if your charger contains secondary safety mechanisms that prevent current flow to an unintended source.
Generic made-for-iPhone chargers have recently been pulled from a big box retailer’s shelves because of the harm it caused and its potential for greater harm.
Even Apple’s customer support page suggests against extended skin contact and moist environments with the USB charger connectors that are plugged in to a power source. A malfunction in a branded charger is possible.
So, the answer is yes, a real danger exists, and this is supported by UL research and non-profit associations such as the CCCA (see article in blog regarding CCCA’s remarks on this danger)