The SSB is fixed. At least mostly fixed. After exhausting every other possibility, the technician returned the theory of inadequate ground. He connected a new wire to ground in parallel with the old one and the transmit current jumped from 10 to 15 amps.
The primary ground is a strip of copper ribbon about 4 inches wide that runs from the antenna tuner to the through-hull grounding plate. The ribbon makes a ground connection and also provides a ground plane for radio propagation. The ribbon is old and worn. I found fragments of it in the bilge. Perhaps it was damaged by the work being done or by the lightning.
Anyhow, the SSB is good enough for now. I'll plan on replacing the copper ribbon myself. Meanwhile, we can leave Rockland tomorrow morning. The weather report sounds good. :)