The only way I would consider flying in the day of the cruise would be in a situation where you could catch up to the ship on land.
We saw this happen on a Windstar that sort of worked its way down the French coast on the way to Lisbon. The ship went out to sea each night and then back into the next port which was probably a 15 minute drive on shore (we joked that we had the same tour guide three days in a row, she just drove up in her van.)
Anyway, the morning after our first night on board we watched a cab pull up and a rather harried couple get out and come on board. We later chatted and found out that they'd had travel woes and had missed embarkation, but all they had to do was get a hotel overnight and then catch a cab to our first port the next morning. No biggie, and the only thing they missed was the welcome reception, but I wouldn't want to deal with rounding up transportation to another island at the last moment.