Leila,
to be safe, I'd say to pack both, I think the code is "no open toe shoes for men". As a rule, i usually wear dress shoes for the a-la-cartes, it simplifies things, as you don't have to worry about how strict they are on the rules.
If you want to be sure, you can send them an email at jventas@ensenachos.co.cu to see what they say.
as for the Catamaran trip, when we were there back in September, it only ran on thursdays so we planned for that. It's an amazing trip, snorkling is awesome.
i think the Melia is about 10-15 minutes drive away from there, though I'm not sure about day trips there, again, use the email address above to see if they have any policies or offers on it.
Hope that helps. If you have any more questions, please feel free to ask.
Hope that Leila, Just a Dad and Jennifer, you all have a wonderful trip there, can't wait to hear how it is now, 6 months after we've been there.