I occasionally get ocular migraines, which though generally harmless, give a different sort of visual impairment and very little warning. It is not painfull, you vision just develops large sparkly blind spots. They can prevent reading, for example.
The worst I was ever hit was leading a difficult pitch on rock, 100 feet off the deck standing on pretty much nothing. I could not see enough to procede, so I just stayed there untill it abated a little, and then felt my way to the top. When I got to the ground a juice pack solved the problem in minutes.
On a boat I would have faced a similar challenge; figure out how to stabilize the boat and then try to treat the problem. Yes, big type would certainly help, and I keep several spare pairs of glasses on the boat.
Thanks for sharing.