I've been out in the field with a student who had a severe hearing impairment.
This was a fieldtrip that I ran with some American colleagues, and we wrongly assumed the student would understand American sign language. The student had a field assistant who could communicate with American sign language, but it turned out the student didn’t understand American sign language. We had to think on our feet and develop further mitigations, whilst we were actually in the field. The strategy we used in the end was that the allocated field assistant was a prolific texter on a mobile phone. The assistant basically used their mobile phone to transcribe very quickly the key points that the speaker was delivering so that the student could see the words on the phone screen. This was the communication that worked for the entire day and enabled the student to engage, feel involved and do the activities.