I think it's a bit much to say you wouldn't set foot in Mexico - that's a bit paranoid. There are a few towns, most near the US border, that I wouldn't visit in a fit. They are out of control with violent crime, for sure.
But south of the border things are a lot better. I just got back from three weeks travelling (by bus, alas, not boat) the Pacific coast around Puerto Vallarta, and travelled inland to Guadalajara. Felt totally safe. I have travelled for weeks at a time through Mexico several times in the past three years and I've always felt fine.
To be perfectly honest, I saw much more gun violence and felt less safe on my trips to Washington, DC. (I still like the city, it just has gun problems)
As to how adviseable it is to buy an ageing catamaran and sail it home, I have no experience in that area. Oaxaca, where you are going, it totally beautiful, though, and would be a lovely place to sail.