There is an app that got good reviews but I only started using it, called STAmina

STAmina Apnea Trainer
BY FREEDIVERS, FOR FREEDIVERS TO STAY UNDERWATER FOR LONGER. ► Helps with physical and mental aspects of improving the ability to hold your breath ► Over 50k+ users Hold your breath longer underwater with the best freediving apnea trainer – STAmina Apnea Trainer. By freedivers, for...
apps.apple.com