Ah, That may not be in -rc3 but is in master and will go out with -rc4 over the next few days.
If you're in a rush and don't want to wait for the next beta, if you're feeling slightly brave you can load "master" by pressing Ctrl-Q from the MP's Initial Setup >> Install Firmware screen. you shuld see the version change to "APM:Rover V3.2.0-dev". Alternatively you can just wait a few days and -rc4 will be out. The frame class change doesn't affect behaviour that much.
The only other advice for Boat users is that ideally the flight controller should be mounted near the point that the boat rotates around. So I think on most boats, the motor is at the back and if you turn, it tends to turn around a point close to the front/bow of the boat. So basically, mount the flight controller near the front if possible. It doesn't make a big difference but if you have options of where to mount the controller..