It depends on what you are looking for. In the Baja area Los Cabos or La Paz would be my option, in the Center, I would go for Guanajuato or San Miguel and Southern Part my option will be Merida or Campeche, but again it depends in what your preferences are.
I used to think of the Yucatan Peninsula as southern since it is the furthest from the northern border of Mexico. Then I visited there and was surprised to discover that I was still at almost the same latitude as Guadalajara. Actually, Merida and the Yucatan are eastern Mexico.It looks like it is north of Guadlajara which is not considered souther Mexico..
We looked at renting in Progreso. The owner, a French Canadian, said part of the year the water was crystal clear but when the northern fronts started coming in it would get murky.I have seen the see how Hound Dog describes it as I was with him and several other people, I guess it depends on the time you go there. We have been there in the summer and November never in the inter so maybe that is the difference.
We went swimming in the gulf many times and never had oil on us or saw it on the beach either. On the other hand we were on the road south of Mahajual and the place was covered with junk, it al depends on the currants , the wind and how long the last hurricane had gone through.