When to visit
If you're flying into a camp in Moremi or the Delta then the season won't make much difference to the access; virtually all the camps open all year. The Okavango's camps used to close down during January and February, but now most keep open throughout the year, despite being much quieter during the green season (December to March). The exception to this has been the January to March period in 2002, when many camps closed down due to reduced bookings from America after the tragedy of September 11.
In terms of vehicle access to the various game-viewing areas from your camp, the rains are often less of a concern than the flood, the peak of which usually lags behind the rains by several months (depending on how far up the Delta you are). However, generally there will be the most dry land from about August to January.
If you're driving yourself into Moremi, then understand that only a restricted area of the reserve will be accessible to you, and the wet season will make this access more difficult. As the rains continue, their cumulative effect is to make many of the roads on the Mopane Tongue much more difficult to pass, and it simply submerges others.
The direct track between North Gate and Xakanaxa has been particularly problematic recently. It was waterlogged for traffic for virtually all of 2001, due to a combination of rain and floods. Thus only the experienced and well-equipped need even think about driving anywhere through Moremi between about January and April.
Most visitors who drive themselves come to Moremi in the dry season, between around May and October. Then the tracks become increasingly less difficult to navigate, although even then there are always watercourses to cross.
Flora and fauna
The flora and the birdlife are definitely more spectacular during the rains. Then the vegetation goes wild, migrant birds arrive, and many of the residents appear in their full breeding plumage. So this is a great time for birders and those interested in the plants and flowers.
The story with the animals is more complex. In most of Moremi there is water throughout the year. Higher water levels here simply means less land area available for animals that aren't amphibious. However, around the edges of the Delta, animal densities are hugely affected by the migration towards the water from the dry central areas of the country.
Thus game viewing generally gets better the later in the dry season that you go … although this trend is probably less pronounced in the centre of the Delta than it is at the edges.
The water levels will affect the activities that you can do whilst here. In some areas mokoro trips are possible only for a few months every year, when water levels are high enough. Similarly, the game drives from a few of the camps are generally possible only when water levels are low enough.
The cost of some camps in the Okavango varies with the season. This depends very much on the company owning/marketing the camps, but in general you can expect July to October to be the highest cost level; December to March to be the time of lowest cost; and November, April, May and June to be pitched somewhere in the middle.