Children who have a hearing issue need to be considered carefully from both a safety and an instructional point of view.
Safety - ensure you have a planned and rehearsed signal for gaining the attention of the child, asking them to move to a specific place or starting or stopping an activity. Also make sure you have a planned emergency signal and that they understand what to do if they observe the signal.
Instruction - work out how you are going to communicate with the child. You may sign to them, use keyword signing (simply learn the signs for a few simple words such as 'stop, go, look, bubble, fast, slow, up, down' etc), use gesture, assign a buddy, use a waterproof notepad on the pool deck where you can write and draw pictures or use a combination of methods.
Obviously visual cues are vital for a child with a hearing loss who is participating in swimming lessons in school as part of physical education. Ensure that any other teachers who work with the child know they do not hear, and that you use a common instructional method to gain consistency.
Note - an important consideration is whether there are other safety issues for the child, such as being allowed to get water into their ears in the case of children who have grommets (tubes) inserted. These children can generally swim with permission from their doctor, but may need to prevent water from getting into their ears.