On my boat (2003 bay stealth) the main livewell has 3 parts to it.

There is a hose coming into it that feeds the water via an electric pump.

It also has a big drain in the bottom with a plastic riser that you can push into the drain to plug it off. The riser extends up to near the top of the livewell and has a screened top. If the livewell gets too full it will go into the riser and drain.

Both of the above make sense and seem necessary for basic operation.

Today I was looking at the boat and noticed a clear tube that has a fitting near the top of the livewell. It has a valve where you can open or close it. I followed the tube and it goes out the back of the boat, on the outside of the hull, and connects to something that I thought was the speedometer pickup on the bottom of the hull. Any idea what this is for?

The valve was closed so I will open it next time I am on the lake. From the looks of it maybe water is forced up into the livewell when the boat is in motion? Maybe it is a backup in case the electric pump goes out?