I thought the following might be useful for anyone planning a trip round the Outer Hebrides this summer in which they're intending to wild camp most of the time. If you are staying in official accommodation your showering and toileting needs will be sorted for you, but if not then make a note of the info below. And if anyone has any more to add, please leave a note in the comments. The prices are from the summer of 2012 and are correct to the best of my memory.
Barra - toilets at the village hall in Castlebay (not open until about 9am and closed early evening) and also at the ferry terminal in Castlebay (but only open when the waiting room is open around the time of ferries). Showers in the sports centre in Castlebay (£1.05)
Vatersay - toilets in the village hall. Walk round the outside of the building to the left.
Eriskay - toilets and showers at the ferry port (24 hours, showers are £1)
South Uist - toilets and showers near the tourist office in Lochboisdale (showers £1)
Benbecula - toilets and showers in the sports centre (showers are £1.05. The centre is closed on Tuesdays)
North Uist - toilets by the harbour and also in the museum in Lochmaddy. Showers at the Lochmaddy Hotel but I don't know the price.
Baleshare - no facilities
Berneray - toilets are at the ferry port (24hrs). Showers and toilets by the harbour about a mile from the ferry port (closed at 9pm, showers £1)
Harris - toilets at the ferry port in Leverburgh (24 hrs). Toilets and showers in the bus station in Tarbert, not sure of the opening hours. (Showers 50p)
Lewis - toilets and shower at the Kinloch Historical Centre in Laxay near Stornoway (24hrs, showers 50p), also at the sports centre in Stornoway (£1.05). There are also public toilets by the yacht harbour and car park but there is a charge and they are not open 24hrs. There are toilets in the ferry port where the ferries leave for Ullapool.
St Kilda - toilets at the back of the shop
Also the ferry from Oban to Barra has showers, but the Stornoway to Ullapool ferry doesn't.