Sibling pups fighting

Question
3 weeks ago we adopted a male and female lab of 7 months. This is their 3rd home.  We had them sterilized 2 weeks ago. Increasingly we are witnessing fighting/playing which is really scary and very violent and aggressive. It seems especially so when they are around water (we have a pool and a fish pond).  The male jumps in and "challenges" the female and then chases her out of the water and they end up biting each other really badly.   When they are on their own with me they are as good as gold and calm and well mannered.  The male seems to be be acting up. Is he challenging me for top dog position?  We have been told it isn't a good idea to get puppy siblings. We are also battling to train them because the feed off each others excitement. The female tries really hard but the male is always challenging us.

Answer
Are you sure it is really fighting, and not just play?  Real fighting, especially a male attacking a female, is unusual.  Are their hackles up and there are taking short, quick bites at each other?  If not, it may only be play.  I seldom suggest private trainers, but somebody more experienced needs to see them to understand what is happening.  It willbe difficult to correct if it is fighting.