[Dan Wilko]

Main muscles involved;
Hamstrings for flexion of the leg around the knee joint (foot moves towards arse),
Quadriceps for extension, leg straightening.
Calves move the actual foot.

Not sure what CV activities would benifit, but i imagine calf raises would help.
Can be done at home or at the gym.
Go to some stairs, balls of your feel on the step, and just raise your heels.
Do this as many times as you can.
To add resistance you could do it whilst holding dumbells.

Also to help prevent cramp make sure you are well hydrated before your dive.

hth

Dan