For a sensor to detect rudder angles or dive plane angles, Minnkota and Lenco have the setup built into the trim tab actuators/switches. The system is a little pricey though. The Minnkota actuators with a 4 inch stroke are $259 for the actuator alone, and another $80 for the switch with LED graph.
As an alternative, it should be possible to use the idea suggested by Alan. He mentioned a magnet coupled to a simple "string" of magnetic switches, wired to an LED display inside the sub.
In my application, it could be set up on the linkage controlling the dive planes or rudders. The number of wires would be determined by how many magnetic switches are used. I'm thinking 5 would suffice. One for center and two on each end for half way and full stroke. The whole thing could be potted in epoxy making it waterproof. Replacement of any individual part would be fairly inexpensive. The magnetic switch "strings" could be fabricated and kept as a "spares", the magnet attached to the linkage with a screw, and a small epoxied thru-hull. The internal display is the same, Low tech and cheap to build, easy to replace. Keeping it simple is the key to low cost and reliability. I bet you could even find the magnetic switch "strings" and internal LED graph parts as off-the-shelf items. You'd have to seal them up ( potting in epoxy ) your self and make a mount specific to your application, but replacing a blown part would be easy.
Frank D.