EMC, there is nothing funny about hearing the call to prayer underwater, I would imagine it like astronauts in the outer space who perform prayer, timely.

Divers wear diving computers around their wrest (with compass and digital watch attached), they know the prayer time ahead, and will perform it when time is up.

Just a note, technical divers stay underwater for very prolonged hours (think of pilots on long flights), they might not want to miss on their prayer(s), and some pleasure divers DO pray underwater in their own way.

Remembrance has no preconditions expect having an open heart..:)