• Kick set: hand lead, on your back, freestyle kick or butterfly kick (butterfly kick is preferred), no board, no fins.
• One-arm butterfly drill, toss in one full-stroke butterfly per lap.
• Swim more breaststroke.
• Swim breaststroke with a butterfly kick.
• Swim a lap alternating between breaststroke and butterfly. It's a coordination test.
This the most effective way to increase your core strength.
So develop it
Increased core strength benefits everything you do, from standing and walking to golf and tennis, everything.