Not Medical Advice: You can not go swimming after a microdermal until it's completely healed. It will take at least two months or in some cases a year.
As per Axiompiercing.com, piercings go through three stages of healing:
1. Acceptance - The stage in which your body will present a number of signs to tell you that you have a foreign object in your body and you need to do something about it. Often these signs will include signs that usually point to a possible infection. Including redness or discoloration, swelling, heat, etc... Once the body realizes that you are not going to remove the jewelry aka foreign object, it will begin the job of producing a layer of tissue around the jewelry to close the wound.
2. Healing - Once your body has accepted the fact that the jewelry isn't going to be removed it will begin forming a tunnel of tissue around the jewelry. The body forms this tissue starting at each puncture wound and then slowly adds additional tissue until the two tunnels connect in the center. Often during this period the piercing may appear to be healed. Though there will usually be some signs of new skin growth like the lymph discharge, there is days and weeks when it may not appear. Also often the piercing is not tender at all. However don't be tricked the piercing is in fact an open wound and a pathway for foreign pathogens to enter the body. Thus it is still prone to infection.
3. Strengthening - During the finial stage the piercing is no longer an open wound but the piercing tissue around the jewelry is thin and fragile. Meaning that the piercing tunnel can tear if there is abuse to the piercing and also it is prone to closing if the jewelry is removed for long periods of time. During this last stage the body will add additional tissue and increase the thickness of the piercing tunnel to make it stronger and less prone to damage. This can take up to a couple of years.
Microdermal piercings or single-point piercings, are piercings installed on flat surfaces of the body.
In dermal piercing, the anchor that holds the jewelry is inserted under the top layer of the skin, into the dermis. This can be done with a needle or with a punch.
Learn more about dermal piercing including after-care, and risks at Tatring.com.