Hi @BillyBudd, welcome to the forum 
There were compass spinning issues in ArduSub 4.0, but they should be fixed in ArduSub 4.1 except in cases where there’s a poor gyroscope calibration, or a faulty gyroscope. See this thread for some additional discussion.
@schleitauche since this thread has now been bumped I’ve edited your post to embed the youtube video directly, so people can view it without needing to leave the forum.
I particularly enjoyed the shot of the ROV through the ice