Just for fun, I tried to think through the best approach for getting as many species as possible in a day, and came up with this:
Run several moth sheets with miscellaneous lights (UV, mercury vapor) starting at dusk the evening before; at midnight of the Big Day, start photographing everything that’s on them already or comes to them. Intersperse with roaming about for night insects not attracted to light and other night creatures. Collect water samples from nearby water sources and alternate the outdoor patrols with looking for microorganisms under a microscope (so as to get some of these without using up daylight hours). Hopefully catch a nap during the night so your eyes won’t burn out before the day is over!
When dawn approaches, start hiking. Try to record bird calls. As it gets light enough, go for anything that moves—birds, herps, mammals, arthropods, etc. When there are none of these to be seen, photograph all the plants, fungi, lichens, slime molds, and so on that you can find. Check under rocks and logs, sift through leaf litter, etc. Hop around from one location to another, trying to balance the lesser efficiency of time spent travelling against the benefits of varied spots. Altitude variation would be great, if possible, and some riparian habitats.
At low tide, get to the shore and photo all the beach/tide pool creatures, algae, seashells, etc. you can find (yes, you’re starting near a beach—a tropical one!). Next, snorkel the shoreline photographing all the fish, coral, sponges, echinoderms, and so on. Then do a dive or two to get the species that are down deeper. Return to land and continue working on plants/everything that moves until it gets dark, and return to your moth sheet/roaming/microscope approach (working a different area than the previous night) until midnight rolls around again!
Of course, to really optimize like this would involve finding a good locale ahead of time, with multiple habitat types on land; good tide pools, reef, and maybe muck diving quickly accessible from the same area; and preferably at least two base buildings, one per night, where you could set up a microscope, access water for microorganisms, and get a lot of insects at your moth sheets. You’d also need good equipment, including a microscope setup with the ability to take photos, a good underwater camera (the better the camera, the more species you can get from each photo), snorkeling and scuba gear and access to a boat, moth lights, etc.
I’m sure adjustments could be made to get taxa that I haven’t considered. Also, a group of specialists in different fields could take this on together, helping each other see species that one observer might overlook, or could act as a support team to help a single observer get as many species as possible.
More theoretical than practical, but fun to think about! I wonder how many species a person could find in one day, in a biodiverse spot, with this approach?