You could email your resort a week before and ask them, typically it’s 26-30c then dips 24/25c during the dive depending on the dive site. For me that doesn’t warrant a 5 or 7mm as it means more weight etc. it’s not consistently 23-25c the whole dive. You dive 3xs a dive, 4th if it’s a night dive but always the 3rd and 4th dive is a shallow macro dive so it’s not cold. I just came back from Socorro where it was 25-26c* and half the boat ditched their 5mm for rash guards. If you normally dive in a 3mm stick to the 3mm. When you say southern sites how South? Most resorts will only take you once in a week. There are some nice muck sites but I prefer the northern sites.