Thank you for your question. If you are 3 weeks post-treatment and fully healed you are perfectly okay to go into the water. You will only need to protect the treated area from sun exposure to minimize your risk of pigmentation issues. I recommend addressing your concerns with your treating physician. Best of Luck!
If the skin is healed from the laser and you are more than 3 weeks after treatment, it is perfectly fine to go in the water. You must protect from the sun or the future laser treatments will need to be delayed. If you have gotten tan or sunburned, you must wait at least 3 weeks from that time for the next treatment. There is increased risks of burns and pigmentation problems if the laser treatments are done too soon after sun exposure.