The first beds I built were 8" and I used pine.

When I remodeled my backyard this summer I redid all my raised beds and this time I used redwood. I opted for 12" beds even though they are on dirt. I just like having a deeper bed even though SFG tells you that you can grow things fine in half that much dirt.

Regarding wet/dry...the biggest problem I think you'll have is your fasteners. As the wood dries it will shrink and your fasteners may become loose.

Thanks for the advice! I hadn't thought about fasteners coming loose.

Don't know what area of the country you're in, but if you have a couple months before planting time, why not put the boards in your garage or basement and let them dry a bit before making the beds.