About 4 years ago I started experiencing severe nausea during diving to depths of 50 feet. I never get nausea on the surface except just after a dive. On my last dive I vomited under water. I never get motion sickness either on board a boat or flying an airplane. I am an 8000 hour pilot and have done aerobatics and regularly fly in very turbulent air with no nausea problem.
All dives are before lunch.
However, the nausea started well before the heartburn.