1. Are you free... [Saturday night?] 2. Are you doing anything... [Saturday night?] 3. Let me check my calendar. 4. (informal) Do you wanna... [see a movie?] 5. (formal) Would you like to... [join me for dinner?] 6. I'd love to! 7. Sounds great! 8. I'd love to, but I have another commitment. 9. I don't think I can. 10. Maybe another time.
Always remember that you have a fallback if you're in an unmanageable depth or you simply can't move your limbs - floating on your back. Don't flail around or start breathing quickly if you can't swim, it is not advised; simply lie back as flat as you can, and let the water help you regain your composure.