The single most useful thing to understand about the waterfront is that Avenida Rafael E. Melgar is priced for people who will be gone by six.
The shops between Punta Langosta and the ferry pier are a duty-free corridor: jewellery, watches, tequila, silver, branded pharmacies. Several are chains that appear in every Caribbean cruise port. The restaurants directly on the water are priced accordingly, and many quote in US dollars.
Walk inland. Crossing Avenida 5 already changes things. By Avenida 10 or 15, and certainly out past Avenida 30, you are in the town that does the island's actual business, and the same plate of food costs a fraction of what it does on the seafront. Menus are in pesos, which is also how you should be paying — dollars are accepted almost everywhere at a rate that quietly costs you ten to fifteen percent.
This is not a warning against the waterfront. A beer on a terrace watching the ships pull out is worth paying for. It is a warning against assuming the waterfront is the only option, which is what the pier layout is designed to make you assume.