In December, it was 80 Pesos for 1-4 passengers from Casa Del Mar to Centro. I do not believe that there has been any change, and should be exactly the same from El Cid or Park Royal. There may (will?) be a surcharge if you take a taxi from the queue waiting for passengers at the International Pier, but since you'll be diving in the mornings, that shouldn't be an issue. Taxis were STILL quoting a 10-1 exchange rate (80 Pesos or $8.00) in December. Now, with an exchange rate of 15.00 Pesos per Dollar at the Banks, 80 Pesos only cost you $5.33, so exchange your dollars and ONLY pay taxis in Pesos.
BTW, be aware that the El Centro zone only goes back to Ave 35 (I think); if you are going to taxi to some of the Restaurants or other places farther back from Melgar, like Casa Mission, El Moro, etc., the fare will be higher - maybe 100 Pesos, if I recall.