A powerboat is a boat with a strong engine, built to go fast on the water — the kind used for racing. The “power” is the motor, not sails and not oars. In careful use it is the racing machine; though actually some people stretch it to any fast motorboat you might see tearing across a lake. It still is not a ferry, a fishing boat, or a quiet sailboat. Written as one word. You meet it in sports reporting and holiday talk more than in everyday city English.