Snorkeling tour in Ko Phi Phi, how to select a tour program?

I'm gonna reserve a sunset tour program in Ko Phi Phi for the day of arrival!
Is it ok not to reserve in advance?

N plz recommend any knowhow to select the program:)

Day tours that include snorkelling at Phi Phi are easily booked once you arrive. No need at all to prebook.

thx, samara400!!