Hardwood flooring
As mentioned below, teak is not affected much by moisture. It's natural oils resist decay and it's structure keeps it from swelling or shrinking with changes in temperature or wetness. It's also pretty hard, and stands up well to wear. That is why it is the wood of choice for decking and cabin soles. Other hardwoods used for floorboards will have problems. Oak, for example, will shrink and swell a lot. That's why oak flooring in houses is generally made in thin strips. Wide oak boards might "move" an eightth of an inch or more with changes in humidity. This would leave gaps between boards or crush the edges into ridges. Thin strips swell and shrink smaller amounts, and so reduce the the problem to an acceptable level. Oak also turns black if it gets wet. These two reasons are why oak ( and wood in general) isn't used a lot in bathroom or kitchen flooring. (You were planning to keep the water out of it by not using any nails or screws and by varnishing all six sides of each board ?) Other hardwoods such as birch, poplar, or cherry will each have their own issues. All of these have probably been used by somebody at some point. Douglas fir, too, for example. But to keep fir swollen so the deck wouldn't leak, Jack Aubrey's crew on "Surprise" spent a couple of hours every day wetting it down and washing it with holystones. That kind of process would probably result in a mutiny from contemporary crew. Teak veneer, protected by several good coats of varnish, should last a good while. We're replacing our floorboards now because the varnish protecting the edges of the plywood cracked, let in moisture over the past 24 years, and the wood is delaminating. The teak still looks ok, but it's not worth re-gluing to new floorboards.