Some random upgauge while I was browsing through FR24:
CI154/155 (to/from TPE), normally B738, upgauge to B744: https://www.flightradar24.com/data/flights/ci154
KE751/752 (to/from ICN), normally A332/A333, upgauge to B744: https://www.flightradar24.com/data/flights/ke752
OZ120/121 (to/from ICN), normally A321, upgauge to B772: https://www.flightradar24.com/data/flights/oz121
MU529/530 (to/from PVG), normally A320, upgauge to A333 on the 5th: https://www.flightradar24.com/data/flights/mu530
MU719/720 (to/from PVG), normally A320, upgauge to A332 on the 5th: https://www.flightradar24.com/data/flights/mu720
VN340/341 (to/from SGN), normally A321, upgauge to B789: https://www.flightradar24.com/data/flights/vn341
(Normally both KIX flights to/from HAN and SGN are on B789)
VN346/347 (to/from HAN), normally A321, upgauge to A359: https://www.flightradar24.com/data/flights/vn347
PR436/437 (to/from MNL), normally A321, upgauge to A333: https://www.flightradar24.com/data/flights/pr437
LH736/737 (to/from FRA), normally A343, upgauge to B744: https://www.flightradar24.com/data/flights/lh736
HX6682/6683 (to/from HKG), normally A321, upgauge to A333: https://www.flightradar24.com/data/flights/hx6683
KE747/748 (to/from ICN), normally B739, upgauge to A332/A333: https://www.flightradar24.com/data/flights/ke748
There are probably more flights that was upgauge. Some of the upgauge may also be just due to the fact that most flights on the 4th to/from NGO was also cancelled.
NGO spotters will see plenty of widebodies that normally doesn't fly there the next 2-3 days, though.
That reminds me a few years ago when a ship hit the bridge connecting to HKG, and the bridge was closed for a few hours. Even that caused major disruption to traffic to/from the airport.
I cannot imagine there are nearly enough ferries and people to operate them, to even cover 10% of the capacity of the bridge.
Can't they use military amphibian landing boats? I'm sure the Japanese Defence Forces are well equipped. And perhaps they could ask the Americans at Okinawa for help, too. At least then all this military equipment once get used for something useful that helps people.
Asking the US military was my first thought as well (similar to Sendai airport from the 2011 tsunami), but the damage in KIX seems much less severe than Sendai. Maybe maybe not.
There is just not the need. You're talking about a very localized situation which people were stranded, but even then, it's not like KIX has zero resources on the island.
Damages otherwise may look bad, but not crazily severe. There are your typical things like down power line (thus power outages), some wind damages to building (especially those that are under construction/rennovation). Otherwise, the only place that got affected badly by the storm surge is KIX. 90% of Osaka area are already back up and running anyway, along with most of the train network (although with delays, since some of the power line on rail lines were damaged).
Keep in mind how widespread the damages were during the 2011 earthquake/tsunami. You're talking about 300mi of coast, villages that were literally flatten, and also a nuclear incident occurring.