What is appropriate for an evening wedding in Mexico in December on the beach?

I have several choices, my instinct is always to dress up but I have a casual option as well. The choice are:

Structured white strapless cotton sundress w/ black flower print
Black ruffle mini dress with mesh cap sleeves
Sapphire textured (shiny) sack dress with halter neck

Do you have any suggestions for what would be best? Any idea what my husband should wear

I can post pictures if that would help. Thank you!