does the jack screw into the mp3 player?
No, it doesn't screw. It is a normal jack and is compatible with any mp3 devices.
I use it to swim and sometime to snorkel with (when snorkeling, I never went more than 5 feet down, the pressure will push the earplug in your ear and most likely create pain and get stuck). I never had any problem with the jack disconnecting while swimming. The problem I had at the beginning is water entering my ear and altering the sound, just put them before entering the water.
In addition, I had to buy a new set because I lose one of the ear ?hook?. With this new set, I simply glue them (after a couple of swim to make sure they are in the perfect position).
Hoping it helps you, it is a nice product.
No. It does not screw in, just plugs in like regular headphones.