Mode switching issues for a boat

I’m running a fork of Ardurover that is more or less synced to the master branch of the main ardupilot repository plus some custom sailboat modifications (you can see it here Hardware is a Beaglebone Blue with a combined GPS/compass and an S.BUS RC receiver. GCS is QGroundControl running on an Android tablet connected via WiFi.

I was out on the lake today testing a code modification that, while in Acro mode, enables the remote driver to command the angle of the boat to the wind rather than direct control of the rudder and sails. However, the boat refused to transition from Manual to Acro mode unless it was held still. I.e. if it was on the water it would not transition. I tried this with both the transmitter switch and from QGC with the same results. However, if my colleagues and I pulled the boat out and held it still, it would transition to and happily remain in Acro mode after it was returned to the water. This explains why it would happily transition to Acro while on the bench in my shop before heading out to the lake, even without a GPS lock.

Any ideas for what I should be looking at to see what the issue might be?

A little experimentation on the bench showed that this was an arming issue. I.e. it would only switch from Manual to Acro while disarmed. I can’t find a param to control that behavior – is that just expected?