Yes, the cabana bay bus drops you off outside of city walk so you go up the escalator and go through the main security check point that everyone not staying on site would go through. Not that big of a deal, but we preferred taking the sapphire falls boat.
My husband and I found it easiest to just dart across the street at the light (no cross walk) to go to sapphire falls and take the boat. Because there's a light, it's not that challenging. The only thing you'd need to be mindful of is turning traffic. However, if you're with older people/kids who aren't keen on dashing across the street, you can (and "should") take that garden path that starts between the castaway/thunderbird buildings. For a reasonably paced walker, I'd guess it shouldn't take longer than 10 minutes from the start of the path to sapphire falls entrance, but my guess is that if you're with someone who isn't able/willing to safely run across the street, it's probably gonna take you every bit of 10 (maybe more) minutes with a lot of whining on the way. Also keep in mind that the cabana bay resort is fairly big and if you're staying at the tail end of the tower side and need to get to the exact opposite end of the courtyard side to take the path, it's going to add some time on to your walk.
So I recommend just walking the 2 minutes across the street to take the boat. That being said, we did the long/safe garden walkway many times to get to the sapphire falls and didn't find it too terrible, but I did feel stupid whenever I had to go that way. Have fun!