Wide open whenever possible.
A boat at single anchor will naturally swing it’s bow into the wind. I have forward and midships deck hatches that, when open, cup this fresh, constant breeze and send it right through the cabin and out the aft gangway to the cockpit. If the breeze is too slight, I can rig a large funnel from old sail at the forward hatch to scoop more air in. Four ports on each side along the length of the cabin suck in air once the fore-and-aft draft is established. i also have fans throughout. There are no screens, but I have mosquito netting and duct tape that will work. If possible, I anchor a little offshore when there is a mosquito problem. If it gets too bad—and this is very rare and a complete waste as far as I’m concerned—I’ll batten her down and run the A/C. I usually only do this for insistent female guests, and even then not very often.
Last night it was stifling down here in the Yucatan off Progreso. I anchored about a quarter mile out in shoal water and slept on deck. I had a nice night’s sleep and awoke to a warm, gentle rain.