Complex sentence: Only by slowly building experience can one remain calm enough to recall their training should a problem arise, whereas an inexperienced diver (who may be recently trained) will tend to panic when confronted with a similar situation.
Split into simpler sentences: Only by slowly building experience can one remain calm enough to recall their training should a problem arise. An inexperienced diver (who may be recently trained) is more likely to panic than an experienced diver when confronted with a similar situation.