So much faster with a pull buoy

Can anyone suggest drills to help me swim as fast without a pull buoy as I do with one? I work hard at practice 5 days a week and make incremental gains every so often. But my improvement over the past year can't compare to the amount I improve when I grab a pull buoy. So - I need to be working on my body alignment? Keeping my legs high in the water? Strengthening my abs? I try to work on all of those things but I would really appreciate any drill or workout ideas. Thanks!