This sounds OK to me. You can dive into the water. I will watch you while you swim in the lake, then I will drive you home whenever you need .
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HelaI (1 watch) you while you (2 swim) in the lake, then I (3 drive) you home whenever you (4 need).I'll watch [ because you haven't dived in yet / it's a future event / diving isn't planned or scheduled event so it's not 'going to' ]