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?

You should not have to limit your physical activity because of implants, so consider options for placement that are appropriate to your lifestyle. This could be either having the implants in front of the muscle (subfascial is my preference for that) or using the split muscle method instead of dual plane. Split muscle preserves the muscle attachments, dual plane cuts the muscle and has a high probability of animation deformity and implant displacement.

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.