do you mean to tread water or to actually pedal or using a running motion with your legs? treading water like they do in water polo is a bit different to a running or cycling motion…
Well, try both, whatever keeps you above the water. BTW, I am not trying to be ‘specific’ or mimic running action, so use whatever elevates your breathing and HR. I used ‘treading water’, and I guess this is the ‘only’ optin when you get the heavy rock, because it produces upward forces that keeps you from drowning, while running will not produce that much of upward force and it is better used with something that allows you to float instead of a rock which pull you down.
I don’t know about the rock, I picked the one that I could take out I guess 5kg?