It may seem a little daunting to rent a power boat, particularly if you don't have much experience, but don't let that stop you. This was a fantastic way to tour Vancouver.
Call early in the day (even if you want to go later in the day). If you call by 10am, you are guaranteed a boat. We called at around 4pm and got on the waiting list for 6pm when a lot of the boats were coming in. We showed up, got a quick orientation, and were off. We rode around False Creed, around Stanley Park, and out along the coast. We had a 15' outboard, 40-horsepower boat. It got a little rough, but that just made the ride even more fun. We loved the fact that seals came pretty close to the boat, plus we got some great shots of the city skyline.
One note of caution, watch out for floating wood. We were warned, so I was pretty alert . . . and sure enough, I had to avoid a large log. A couple of funny sights: another small boat with about 10 people fishing in it in the middle of the harbor (they couldn't even sit down because the boat was so crowded) and the dog beach near the entrance of False Creek (there were about 20 dogs of all sizes playing on the beach and in the surf).