Hey,
I've been through this a few times with a few different boats.
If there is a lot of old paint build up, the best way is to hire a soda blaster, and have him remove all the old paint. After that is done you will need to fair the keel and perhaps other parts of the hull, then barrier coat (of you like) and apply bottom paint.
If the bottom is in really good condition, you can just scrub the bottom with water and a stiff brush, then apply new pant.
If the bottom is between really bad and really good, then you can scrape the old paint off, sand the old paint off, or use chemicals. It's really up to you.
I can't think of any reason to use soap and water. Either powerwash the hull if it has marine growth on it, or just scrub with a stiff brush and water. Then remove the old paint using your preferred method, scrub and rinse again, then paint.
Good luck,
Barry