Phone travel charger for powering circuits dangers
An unmodified and genuine charger actually made by an internationally renowned cell phone company such a Nokia or Samsung will be very safe under normal operating conditions. Apple ones are great too, but there are many, many counterfeits about. Even if it is top-drawer, I still would not tempt fate by using it when water is about, or any other situation that could involve low impedance connections to a human. 

There are counterfeits and all sorts of junk flying around. Fuses won't necessarily save you from electrocution- if there is an internal breach of the galvanic isolation barrier (say due to bad design, component failure, modification or debris inside bridging the gap) then the fuse (if any) could happily pass enough current to kill you. If the adapter power is flipped it may not even be in the circuit. The purpose of a fuse is to allow the adapter itself to fail gracefully without excessive drama. 

I have personally taken apart several chargers with counterfeit markings (both the company name/logo and the safety agency markings were counterfeit) and those particular ones were extremely dangerous. They would never pass safety agency approval. I would never give one to a kid or to any other human I cared about. I might plug them in for test purposes, but would keep a careful eye on them. Also junky electrically- noisy output voltage and a lot of conducted EMI. 

Aside from reliability and the quality of the power (cheaply made ones may not have OVP, so a failure of a solder joint could render your $1000 phone into trash), the two main dangers are electrical shock and fire. Those dangers are mitigated by careful design meeting safety agency guidelines, listed or approved materials for every safety-critical application and thorough testing. For example, the approval documents will list the approved suppliers of the polyester tape and wire used in the transformer. They must use material only from those suppliers  (who, in turn, follow QA procedures internally). Junk adapters will use whatever is cheapest- maybe even rejected material from top-tier suppliers.