Make sure the cooler does not get hit by the sun. Put as much ice in as you can. Do not stand there with the lid open, looking in while deciding what you want to get out. Do not put anything in it that has not already been cooled- warm items just serve to melt the ice that much quicker. A cooler okay for a picnic just won't cut it for boating use. Pre-freeze as many food and drink items as you can.
Make ice larger than 'cubes'. Small cubes cool things quicker and better (and are good in drinks) but as the cubes move up to block size, they last longer. Find some sturdy containers to make blocks of different sizes and see which ones last however long you'll be out.
I have some of the instrument rear covers that are heavy plastic, and tapered, which allows the ice to pop right out. I also use the cheapo containers that salads or cut fruit comes in at the store. In either case, fill the container half way to avoid the freezing ice from splitting the container, then top it off when frozen.