The water taxi idea, while intriguing, will only last until the first drunken gringo falls off and drowns. Plus, pangas are unwieldy to get in and out of. No dock at Sandy Beach (or the Malecon, for that matter) and if you want to go out for a nice dinner, you aren't gonna want to traipse through the surf to jump in a panga. Add to that you are also at the mercy of the weather. When the wind blows... when it's hot and humid... people want their AC! Just sayin'...
Roberto's comments about working in Mexico as a gringo are spot on. It's VERY tough. I know I wanted to do charter fishing and looked pretty hard at it. About the only way to do it easily is to marry a Mexican and run the business in their name. Even then, the point about competition is also spot on. They can run you out of business and out of town. If you get over those hurdles, you still have the ones the Mex govt. throws at you with taxes and the banking aspects.
I would firmly suggest you meet with a gringo who actually lives and runs a business in Penasco and hear their story. It's amazing anyone even tries to do it.