I was going to post something on the story about underwater robots, but I can't because of this wacky certification mechanism. Anyway, I was going to write about Attitude Heading Reference sensors, which typically combine a three axis flux gate magnetometer, rate gyros and accelerometers. They do internal filtering and output angles and angle rates for yaw, pitch and roll. Two small sub-$2k devices on the market that we are currently evaluating:
InertiaCube2
3DM-G