What about a tank on the dock and pumping water out of the lake and a overflow tube. That is what they use at Greeson and keep lots of bait in. I saw a guy on TV using a big plastic trash can doing this.

I use a six gal. bucket with holes drilled in it about four inches up. That way in I pull it out and take it with me I still have some water in the bottom. I also have a wash machine tub that I put a lid on it. You can try it out if you want. I'm not using it any more.