Sally, they do grow in warmer climates and can do very well - they are pretty common in coastal SoCal, which shares similar zones. However, your high summer humidity may be a limiting factor. I'll let some of the regulars that live closer to you and may also deal with similar conditions recommend specific choices.

Two I might consider would be Clematis paniculata (the true species, not the misnamed sweet autumn clematis), a new Zealand species or C. x cartmanii, an evergreen hybrid of C. paniculata and C. marmoraria, another NZ species.

Why not any of the viticellas and perhaps even clematis texensis the species (native to hot and humid Texas).

Other points to consider are to plant the clematis in amended soil (you may have to continuously add organic material and mulch since sandy soils tend to chew up organic matter pretty fast), keep them consistently moist but not sitting in water, and plant where they will get morning sun and afternoon shade.

I grew jackmanii and one other I forget the name of in pots in Naples, FL (zone 10b) year round. When I got tired of them blooming or if they stopped or slowed down, I'd hack them to the ground and they'd start all over again! It's probably because I didn't know what I was doing, but they bloomed year round....

BRUSHWOOD Nursery has Clematis listed for Z 11, has anyone taken a look and does any of it look familiar ? for here in Pinellas ? I may call them and see what they say, seems a stretch for this micr-climate we have here, we are so humid.. and they are not cheap.... I was really hoping someone from here in Pinellas would be able to chime-in with their experience.... I know roses are sensitive here and some do thrive and some are just dogs, Thanks folks, sally