I sure hope Cadiz is as sunny as it was in 2007. Think shorts and light shirt. We just walked around, some of the folks that put on warmer clothing were tying their jackets around their waist because the weather was so nice.
As for me, since we're going to the UK as well, I'm bringing a pair of shorts with me for cruising around, but khakis/casual pants for traveling on port days. I'm bringing a hoodie jacket for layering, and some t-shirts and button shirts for layers underneath. Oh, and a cap/hat just because I wear weird hats.
I've done three Aprils now (in a row) in Spain, France, and the UK packing that and I've been good. In fact, London was beautiful last year and we didn't need the jacket too much. Paris, on the other hand, was C O L D the first few days, and I was bundled up. Later in the week,
Disneyland Paris was so nice, we were walking around in shorts and a light shirt. Barcelona, because of how it is, the "open" areas were nice and sunny, but going into the Barri Gothic, some of those streets haven't seen the sun for a while..so it just feels colder - and with wind..it can be brisk.
Keep in mind, 72 is a good light shirt temperature for me, and 68 is definitely time for a jacket. Layers are always a fabulous idea.
-Steve