Hi Lottie,
A simple answer is "it depends."
"Depends on what?", you may ask.
(Or you may still be mad at me for teasing you in another post; but I digress...)
It depends on:
What is your skill level, what is your activity, and what is your environment.
My recommendation is that everyone practice for, and be "in shape" (fit) to dive. It's a journey. Do not get complacent with where you are.
Running, weightlifting and stretching is just the beginning. If you don't have your legs in shape to dive you run the risk of injury, just like with any other potentially strenuous activity.
I like to practice with mask, fins, and snorkel.
I practice water skills that I may be called upon to use.
Whether or not you plan to explode into action, a diver needs to be ready for that possibility.
Flutter kicks, deep kicks, dolphin kicks, frog kicks, or just finning;
(my resting stroke, where my legs remain straight, and my fins move like a whip kick; down and whip around like a boat propeller) ALL are needed at one time or another, depending on what? (See above)
As for fins; I am biased.
I grew up in So Cal; I learned at an early age that the ocean is not my friend.
We used UDT fins only, because they were the stoutest fins we could find.
We trained and developed ourselves to manage the surf entries, waves, and surges that were a regular part of our diving.
We were able to get ourselves out of whatever we found ourselves in.
A friend has further info if interested: http://www.myspace.com/udtfins
To me, there is no substitute for a good stout fin, and muscles in your legs.
Best of luck.
ASW