but I know people trying to sell something will always make it sound easier/better than it is. www.custombuiltspas.com
I think you have answered your own question. Sure it is possible to build your own spa, but a correctly built one that will perform so that it is enjoyable to use is a little more technical than "make a waterproof hole and fill it with water".
Of course, once you buy this guys book (hint, hint, hint), you will want to rush out to buy the books from all the others who are so concerned about helping you...my book will teach you how to buy property for no money down, and you will be worth millions in just one year; my book will teach you how to buy property at tax sales and you will be a millionaire in a year; my book will have you buying "distressed"properties and flipping them for millions of dollars; even Donald Trump wants to teach you how to be a millionaire with ease. Gee, Donald, did it help to have a daddy who owned 20% of Manhatten?