Yes it could, but it is unlikely. Hydrocoeles (unlike varicocoeles, which are varicose veins of the scrotum) do not increase the temperature of the testicles. Nevertheless, any operation would always run the risk of a complication such as infection, which could damage the testicle. This is very unlikely to happen and even if complications did occur, the other testicle is likely to be normal. If you are worried, then it would be a relatively easy task to perform a sperm count. Your GP would be able to arrange for this to be carried out.