If you want to keep minnows alive for any lenghth of time, you have to keep them cool and oxygenated. Look at what the bait shop does. Keep the water moving by pumping it in over and over. In a boat , you could set up the live well to constantly pump water in to the overflow. Its easier to get a Bubbles air pump and use flashlight batteries or hard wire it. I never buy more than I can use in a day.
The best minnows to use are ones you trap yourself.. They are acclimated to the water you are fishing in.
