Finally leaving on our honeymoon to Playa del Carmen...what to wear??

It sounds like what you're bringing is perfect. You will never want anything with sleeves (but be sure to use lots of sunscreen on your shoulders, back and arms!). Yes, it's a good idea to wear your swimsuit under your clothes if you're going to have the opportunity to take a dip on your tours. For evening, sundresses and capris are appropriate. If you're staying in a resort, you may want a sweater for the air-conditioned restaurants. If you have longer hair, be sure to bring clips or something to keep your hair up... it's so much cooler that way.