I used to swim a long time ago and after a break of several years, I took it up again about 6 months ago. However, my triceps (i.e the lateral part of the back of my upper arms) still get fatigued very easily even after 2 or 3 50m lengths. I'm sure this is because of a technique issue.. what could be the cause and what can I do to correct it?
Obviously your stlye needs to be checked etc.
But also at the end of each lap, hop out and do some push ups with arms close to side of your body (not out wide / as in normal push up position) so that you focus on your triceps...do this several times. It will greatly improve overall fitness and strengthen your upper body especially tri's.
plus make sure you are fully hydrated before and after and diet is getting plenty of magnesium and potassium and vitamin c etc.