Does anyone have suggestions for growing onions in S W Florida?
What varities? When to plant? How much water? etc.
This link will serve you well in answering your questions.
You need to grow what are called 'short day' onions in your area. The link will explain what those are. The site is a commercial site as well so you can purchase plants of appropriate varieties from them if you wish to. I have been ordering from them for a few years now and find they send a ton of plants, way more than they advertise. Also at the link is a place for you to enter your zipcode and this will tell you when onions will be shipped to you. This tells you when to plant as they only ship live plants when it is planting time in your area.
In areas with frosts onion plants usually go out 6 weeks before the last expected frost. I don't know if you get frosts in your area or not.