I used to swim a lot - have been rubbish recently. If you can afford it, you need the waterproof iPod armband and the headphones (and then of course the iPod that fits the band) - they're not cheap but it really kept me going. You need to pick your music tempo though - I speed up to lunatic degrees with the wrong music .
So much of swimming is to do with good technique - if there are adult lessons around, then I'd go for it.