Friends don't let friends book cruise ship excursions. Book them yourself; you'll save a bundle, and be treated like less of an idiot by the operators.
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This the best advice u can get. Boats that have contracts with the cruise ships will treat you very differently than those who do not. The boat will be overcrowded, everyone is in a rush, and they don't really give a crap if you have a good time or not because they have another 3000 people just like you coming in the next day. Other operators will give you much better customer service, because they have to sell themselves to get you to come back and dive with them. They will also typically be less expensive because they don't have to give a huge percentage to the cruise line.