If I have a breast implant under the muscle, would I have to give up swim training or just reduce my hours in the pool?

Thank you for sharing your question.  Once fully healed you should be able to return to your normal training regimen, without restrictions. 

Neither.  As a bodybuilder, I perform breast augmentation in fitness and
bodybuilding patients, and there will be some level of animation
deformity with submuscular placement.  The goal is to minimize it to the
extent possible. Under the muscle implants are more likely to be felt
moving because the pectoralis muscle is attached to the arm.  This will
become less evident over time.