Papas Arrugadas - Wrinkly Potatoes - are a very popular tapa in the Canary Islands. Traditionally they were cooked in sea water. Served with Mojo Verde or Mojo Rojo, or both.
Scrub the potatoes clean, but don't peel them. Place them in a single layer in the bottom of a pan - choose a pan size that doesn't leave too much empty space. Add enough water to cover the potatoes, and then add the salt. If you want to be precise about this, you could measure the amount of water you have, and then calculate what 3.5% of that volume would be - that's the amount of salt in seawater.
Put the pan on the hob, cover it with a folded, clean tea-towel, and put a lid on top. Make sure the towel isn't too close to the heat source. The towel is used to absorb the evaporating steam and ensure it doesn't fall back onto the potatoes. Bring it to the boil, and let it bubble away for 20 minutes.
After 20 minutes, remove the lid and towel from the pan (be careful with the towel - it'll be saturated with very hot water) and check whether the potatoes are cooked by stabbing them with a fork. If they are not cooked, give them a few more minutes, otherwise drain off all but 1cm (.5 inch) of the water and put the pan back on full heat. You want this water to evaporate and leave behind the salt, so you'll need to stand over it and stir constantly to make sure the potatoes don't stick and burn. When the water's all gone, you're done.
Serve the potatoes with the mojo verde dribbled over it and some crusty bread.