I've introduced weimaraner pups as early as 6 weeks of age. Bronko had excellent advice. Start in puddles, shallow water. I like to use the shallows in a slow-moving small stream/creek. A step or two and you're across. Take pup for a walk and and just stroll across. Don't make a fuss, keep quiet, and continue walking. They may hesitate - but will usually quickly dash across the shallows to be with you. From there, gradually larger or deeper water.
When 12 weeks or so, if doing well in walking through puddles or shallow streams we'll try the pond. Let them explore the shallows - no pressure. After a trip or two to the pond, it's swim time. Luckily, I have a son that loves to swim in the pond. He'll wade out, take pup with him and let him swimm back to shore. I sometimes wade in also, and we have the pup swim from one to the other. Then encourage pups to swim from shore to him...just a few feet. Couple times of that and they feel comfortable. If they are also retrieving - combining the retrieve with water really gets them going.
Of course, warmer the weather the easier. Try to get pups accustomed to water before water temp drops in fall.