Infant travel from krabi to phi phi by speed boat

Planning a week trip with our 20month baby, to krabi and Phuket.. Planned to first land in krabi and then visit phi phi.. I have below question if anyone can provide information that would help…
1. Is infant allowed on speed boat if planning one day tour to phi phi island?
2. Do We need to book in advance as our dates are not certain due to rainy days . Can we book day tour one day before of actual tour ??
3. Which one will be safe for infant ferry or speedboat and if ferry what will be the timings from Ao nang pier for return journey and can it be booked one day before
Highly Appreciate your help!!!

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