In Canada, watermarking patient data into a medical image would very likely be an offense under the federal Personal Information Protection and Electronic Documents Act (PIPEDA) or under one of the provincial Personal Information Protection Act (PIPA). In the USA, it would very likely be an offense under Health Insurance Portability and Accountability Act (HIPAA)
Keep in mind that watermarking requires that the information watermarked be accessible, perhaps even legible. Watermarking is not about concealing information so that it cannot be found: watermarking is to be found. That is the difference between watermarking and steganography: steganography is placing the information so that it is not to be found. The techniques used for the two are very closely related.
