I once did some work for a team that did ELF communication from a small autonomous sub to a surface ship for mapping. They had a 1200 bps channel up to the ship for the map data...

(I didn't get to do anything with the sub - I was just brought in for two weeks to give them a way of feeding that data from a Sun workstation on the tracking ship to another station via GSM data (mainly for demo and testing purposes); trivial in comparison to the software controlling the sub, but it was fun getting to go out on the tracking ship when they did test runs)