My initial thoughts are "Eek! This is best OP could come up with for 18 days on a first-time trip to SE Asia?!" Then I decided more info is needed, in order to make appropriate suggestions:

1) OP, where are you from?
2) Are you the crazy-for-beaches type? Previous beaches and expectations?
3) Do you have other interests, or any prefer-to-avoids?

I can't imagine planning a first SE trip like this for myself, nor would I advocate your itinerary to anybody. Just seems like a waste of good opportunity.

If you get sea-sick easily, the boat ride Phuket-Phi-Phi will likely upset you. November water is usually reasonably smooth but the boats they use have a lot of yaw and bump. Phi-Phi stopped being a great place to go about 15-20 years ago. Skip it. Skip KL too (like the city but it shouldn't be a priority on a first trip of only 18 days).