Is it obligatory for an injured person to consult a doctor to determine the extent to which the wound needs a plaster or bandage, and does water reaching the wound cause harm, or is there great hardship in that?

It is not a condition to inform the doctor for the permissibility of wiping over a splint. Rather, it is sufficient to have a strong presumption of harm occurring if water reaches what is beneath it. This can be known by the doctor's informing, or by experience, or by the information of someone whose temperament matches that of the patient.