Learn about resources that can help you decide.
Outboard or inboard? Gas or diesel? Four-stroke or two-stroke? The choice of engine and engine type dramatically impacts your pocketbook. Because many boat manufacturers are allied with engine manufacturers, these companies are always trying to limit your choice of engines to their partners. How do you know what's best for you?
Don't expect the seller of any engine to tell you to buy some other manufacturer's engine. But the engine manufacturers do give you specifications, including fuel consumption. The manufacturers' sites also give you a chance to see the different types of engines available to you. For instance, what are the advantages of four-stroke over two-stroke? Here are links to key outboard engine companies.
Many boating magazines provide useful comparisons of various outboard and inboard engines as well as general boat reviews.
Several independent authorities such as marine surveyors and technicians can help educate you about marine engines (and lots of other topics, too). Try these for a starter:
Marine surveyor David Pascoe offers over 150 articles on boats and boating including a number on marine engines.
OutboardMotor.Net allows you to access the specs of a number of outboard marine engines, making comparison easier. Be aware that this site is part of iboats.com, a commercial site interested in selling boats and products.