Hi,
How about the lead boat with an Infra-Red source, perhaps a vertical array of IR LEDs omni-directional, and the following boats with a primitive IR "eye" akin to that of a nautilus (nautilus belauensis).
The eye could have a vertical slot rather than a pin-hole, since we are only operating in two dimensions.
The "eye" would have ring of sensors that could detect the source direction and with two "eyes" one could determine the distance (well the Dam Busters had a very simple distance device didn't they).
Once the control system has all the directional data, it would be a field day for applying what ever algorithm that best connects the data to the to the rudder. I'm saying this having had experience with my Voice Controlled navigation system for my Edwardian Steam Launch, where I could say "Figure of Eight" and the boat would steer a figure of eight course.
Each boat in the convoy would have "eyes" at the bridge and the Omni-directional source on the super-structure.
The IR source could even be pulse coded so that individual ships could be identified and ambient IR sources ignored.
Anyway, the world is your oyster (or nautilus). Food for thought!
Ian.