Hi and welcome! Kathy and I took our first Olivia vacation to Cancun and our questions were similar. Best experience ever, BTW!! let me try to respond to your questions.
$ or Pesos - for the Cancun trip, we only had $. We had a bunch of fives and ones for tipping, which really came in handy. At the resort, we liked tipping, even tho it was included. Great at the bar. We did take a couple of shopping trips and pesos would have been better then as the exchange rate on the first one was not wonderful and we saved money by paying in cash, not credit cards - better bargaining power. We got some for the second shopping trip. the magic trick is to know how much. Of course, the resort will do change, but again not at at good rate, typically. Depends on what you want money for honestly.
Rx - we ALWAYS put our Rx in the carry on. no exception, the safest choice. ours are the same variety,but would not want to be without them. In terms of worries about customs and such, the question is more when you come home as US customs takes a dim view of Americans buying Rx in Mexico. They are MUCH cheaper there, of course.
I do not know of that excursion company, sorry. Would be early for whales, but cool. I would love to snorkel, if it is available.
We are from Wyoming, but have close ties to the St. Louis area - look us up!
Becky and Kathy