Yes, very easy.
If you're arriving at the Contemporary by car or bus, head through the main entrance doors, past the reception desks on your left, and straight out the door at the other side of the building. You'll see the marina straight down the path in front of you.
If you arrive by monorail, head down the escalator from the monorail and you'll be on the floor where the shops are. Take the stairs or escalator down one more floor and on your left you'll see big glass doors. Go out of these and down the steps outside, and again you'll see the marina in front of you.
The 'Wishes' cruise is wonderful, you should get a tour around the lake first and see the various hotels, then stop opposite the MK for the fireworks. Enjoy!