No, you don't need to put a regular diaper under a swim diaper.
Swim diapers are designed to contain solid waste while letting water pass through, and regular diapers absorb water (becoming heavy and ineffective) in the pool.
Precautions for use：
1. Choose the right fit: Ensure the waist and thigh areas fit snugly (not too tight) to prevent leaks without leaving red marks on your baby's skin.
2. Replace promptly: Change the swim diaper immediately after swimming, and limit pool use to 1-2 hours max to avoid skin discomfort.
3. For water use only: Swim diapers can't replace regular diapers-switch to a standard diaper right after your baby gets out of the water.
4. Check before use: Verify the swim diaper is clean, dry, and undamaged. If your baby has a bowel movement, remove it at once and clean their bottom thoroughly.