If I have a swimming match, I dive in.
If I do my job (swimming instructor), I just jump in. (Exept if a kid needs help, in that case I dive in)
Normal swimming? Just jump. You'll get used to the water.
I either do a cannonball into the pool or I slowly step into it taking in the immense coldness slowly and letting my body adjust to the cold.
I've never had the chance to jump in from a diving board, but I'm afraid of heights so I'll just use the short ones if I do get the chance.