MRT? I would think that your best bet for some decent beaches would be to take the High Speed Rail to Zuoying, and from then a bus to kenting. From Wufeng to Wuri is not all that far, 20 min in a taxi, I think.
Mountain swimming holes? Drive up the valleys in the mountains, keep an eye on the little river in the middle of said valley, and once the water is a little blocked up and the river fairly accessible, you got a spot.