The perimeter track around Grafham Water is a very easy walk at any time of year. There's several large car parks you can start from, we normally use the one just west of West Perry and some cafés.
It's also a cycle track. This had its plus side in that the path is of good quality throughout so good even when everywhere else is boggy. The down side is that you get cyclists all over the place. Luckily they almost all do it anti-clockwise so if you're walking then walk it closewise and you will see them coming.
Apart from that the only other problem is a very stinky intensive farming unit on the north side which you walk very close to.
Here's the route on Google Maps.