Hi, Any advice would be much appreciated.
Every time I swim any distance over 1km I get cramp in the arch of my right foot.
Is there anything I can do to stop this happening ?
Runners require tight spring like muscles often with limited ranges of movements whereas swimmers require good flexibility and often hyper-mobility in the foot/ankle complex for a good kick.
Plantar flexing the foot to point your toes requires the muscles in your foot to shorten to the extreme of their range. This is when they are at their weakest and so must likely to cramp as they tire after even a small amount of work.
As Rob say expect the cramps to reduce as you swim more and your feet adapt to the kicking position. Obviously a more flexible/mobile ankle will help reduce the work required of the muscles in your foot arch. Strengthening exercises should include some that include your foot in the plantar flexed position.
Hope this helps