There was an item about apple drop on Gardner's world on Friday evening, Monty Don suggested thinning to two fruits at each cluster. I personally have always left my trees to sort themselves, and still ended with a good crop. How much water you give is dependent on the size of the tree but it is usual to water any newly planted tree or shrub in the first year, especially in a dry summer. Water will help the fruit swell, and as long as the roots do not sit in water I do not think it will affect the natural fruit fall. My apples were the best they have ever been when we had a really wet summer last year.