The more expensive Super Saver systems are made for big coolers, and worthwhile. All these bilge pumps are, of course, meant to be dropped into the water. They have built-in plastic foam which prevents gunk, detritus, and fish from getting into the pumpworks. Carry several rolls of 36 exposures, and shoot, shoot, shoot. You can always give away duplicates or extras.
Slide film is cheap, so use high-quality film and lots of it.
Make an additional plate to set inside. It will the fish from hugging the bottom when it is angled. Use clothespins to hold the plate where you want it, to keep the fish from running around excessively. When shooting, make sure there are no bubbles on the inside and no water droplets on the outside. The water should be as clean as you can get it.