For speed, petrol / gasoline inboard marine engines power motor boats with propeller or waterjet propulsion. Alternatively, outboards offer easy installation and servicing.
Commercial vessels use medium speed inboard marine diesel engines for ships as well as high speed diesel engines found in workboats and recreational vessels.
Small sailboats are powered by outboards, larger sailboats, sailing yachts use saildrives or inboard diesel engines, sometimes inboard gasoline engines.
Depending on the boat diesel or petrol (gasoline) sterndrives sometimes called I/O drives, offer better manoeuvrability than the conventional inboard motor.
Superyachts / megayachts use high speed marine diesels or medium speed marine diesels in a conventional or diesel electric propulsion system; some use podded drives.