What's your preferred aerobic training for freediving?

[subaquaticus]

I do swimming which is obvious and also roller skating...

This last activity is very pleasant when you are a bit fed up with water or cannot go in water for health reasons (a cold, sinusitis, era aching,...)

any way, since "apnea" is theoretically forbidden during public hours I often swim "forcedly" :waterwork when the lifeguard is not enough lenient...