I’ve seen two main styles of livewell:
1. Single pump systems that have an overflow tube. The tube is stuck into a rubber grommet in the bottom. below the grommet is the drain tube that goes to the transom of the boat. The single pump is near the transom – a second hole – and pumps fresh lakewater into the well. To drain, you pull the overflow tube.
The problem with this style, in my opinion, is that the boat has to be on the water to recirculate. The other issue is that any fresh water coming in is surface water, which during the summer is at 70+ degrees, doesn’t hold a lot of O2, and doesn’t keep the fish as fresh unless you’re constantly adding ice.
2. Two pump systems. This style has one pump on the transom for filling with lakewater as decribed above. The other pump is a submersible/bilge pump inside the well and serves as a recirculation or evacuation. The way mine is set up, there is a simple two-way valve to select whether or not you want to pump the water through the aerator nozzle or out (drain the livewell).
The only downsides I’ve found with that system is that there is more equipment/cost/stuff to go wrong, and the livewell doesn’t drain completely – it leaves about an eighth inch of water on the bottom, which hasn’t been a big deal if I at least hose out the fish smell.
If you’re in Minnesota, and are prevented by law from transporting a live fish, then it probably doesn’t make much difference. Here in Iowa, I’m not aware of any such restriction, so when I go to the big river (assuming I can catch an edible fish), I can keep the fish alive with a cup of convenience store ice and the recirc pump for the 2 hour drive back to the garage, where the fish are still nice and lively.
When I get home, if I remember, I’ll take some pictures of how mine is set up. The only difference between mine and what I described above is that the freshwater intake is inside the well, so if it leaks the worst it can do is fill up the livewell. If I were to have to drill the holes, I would put it on the transom, though.