I am doing bottom work on my boat. The previous owner recaulked the
bottom but did a terrible job. Its a mess. So I have been pulling out
the old caulking - some of it is cotton string other is just 3m 5200.
I was told I could recaulk with just 5200 instead of applying oakum or
cotton just make sure I go slow and get it up into the planks. Is this
true? If not then where do I get the string to caulk it with?

First off, I think your out of your league on this. Are you sure the
bottom of your boat is planked, and not plywood? if planked don't use
5200 PERIOD. Use Dolphynite available at West Marine. When you reef out
the seams use a reefing iron or make one, be careful not to tear the
wood up tho. telling you how to caulk (cork) round these parts is to
complicated for me to explain. (anyone else?) just make sure you paint
the seams before you start corking.
Maybe someone over in rec boatbuilding can walk you thru it. UD