I did the Ridgeway in the blazing heat last summer. Water was definitely a concern. One of the marked taps had run dry and another was nowhere to be seen. But I carried enough just in case so it was ok.
The trail itself varied. There were long stretches of dull, and rock hard, bridleway. But there were a few surprisingly lovely stretches. Smeathes Ridge, Waylands Smithy, around Nuffield, Grims Ditch and a few surprise hills in the eastern end.
Didn't see a soul away from the dog walking stretches near car parks. Had deer and foxes running across my path, rid kites circling overhead and clouds of brightly coloured butterflies.
To be honest I somewhat overestimated it. Wearing boots for the western stretch was a mistake and allowing myself 6 days was a (pleasant) mistake too. Was in the pub by early afternoon most days . Speaking of which the route passed a couple of pubs that had closed down which was disappointing!
Overall, the Ridgeway was a pleasant surprise. Had few expectations of it, but was surprised at how "remote" (wrong word - but can't think of a better one) parts of the Western half felt.
When I camped on Svalbard (July '06 iirc) I was too warm in my down jacket. temps never dropped below freezing, and there was no night-time for temps to drop in. A warm fleece was enough. No rain while I was there. Synthetic insulation would be useful for whilst on boats maybe?