Water is around 830 more dense than air so the effects of drafting are HUGE. It can be worth 10 seconds plus per 100m. Training sessions are designed to make you work at a specific effort for a reason so don't undermine your training by 'swimming-on-feet'. The graph below was taken during a recent squad training session and show the huge drop in heart rate when the swimmer was drafting. You are only cheating yourself so MIND THE GAP!