Both are professional rescuer roles with a legal duty to act. The water they happen in is what changes.
A pool lifeguard takes responsibility for the lives of people participating in a variety of aquatic activities. As a professional rescuer with a legal responsibility to act in an emergency, a lifeguard must be self-disciplined and confident in their knowledge and skills.
Beach lifeguard courses are not for the faint hearted. The training involves running, swimming and rescuing, plus dragging casualties through the surf zone and up the beach using rescue boards and rescue tubes. The courses also cover lifeguard and first aid theory, basic life support and spinal management.