My belly button is just red, and very very sore. Will the sea salt and water work to clean it?

How long ago was it done ? You know you just can't have skin pierced and forget about it, hannah. Yes, it will work but you have to be diligent and keep at it. Shower often, and clean that area with a clean washcloth and soap

If you reads to the right of your post, you will probably pick up some more good tips,

Clean the piercing with hot water and regular soap. I have had my belly button, nose and ear get infected and I was told to do so. It helped a lot.

Sea salt and warm water should soothe the piercing and help it to try and heal as it should. Make sure to not over clean it as it would start to aggravate it. Do NOT add too much salt. If you don't see any progress, you should contact your piercing or doctor and get professional help from there.