There is a distinction to be made here. I assume by seaplane that you are using the term generically for any aircraft that can land and take off from water. There are actually 3 types of aircraft categories here. 1. Seaplane or more commonly called floatplanes. These are small and can only land on fairly still water like lakes or rivers. They look like normal light aircraft but with floats instead of wheels. 2. Amphibians. These are about the same size as floatplanes, but have a flying boat hull shape to land on water and retractable wheels for landing on land. These can land on choppier waters than seaplanes but not the open sea. 3. Flying Boats. These are the big boys that have a fuselage designed for landing on water and don’t usually have wheels at all. Of the above, only flying boats have any real use to the military. The Japanese deploy their ShinMaywa flying boats for maritime patrol and rescue (although ShinMaywa refer to it as an amphibian, it is the size of a flying boat and uses its wheels for manoeuvring rather than landing). There are other flying boats in use for firefighting but none (apart from historical aircraft) in use for civilian passengers. It is difficult to use jet propulsion (although Beriev of Russia tried) because of the ingestion of seawater into the engines and passengers these days expect their air transport to be jet powered. Also landing on the sea is inherently more dangerous than landing on a runway. Any serious kind of chop on the sea surface will make the landing problematic as will any floating debris. The landing distances are also huge as they tend to land at a much shallower angle than land based aircraft. Beriev are still making amphibians for firefighting and a new model was certificated in June 2003. Their site is here: http://www.beriev.com/eng/core_e.html

The large number of airbases built in the Pacific during WWII that became civilian airports after the war eliminated one of the major area where civilian passenger seaplanes may have been useful had history been different. The floats or boat hull on floatplanes/flying boats have poor arodynamics so they have poor performance compared to land based types. The widespread use of aerial refueling in military aircraft made that a faster,cheaper and safer way to refuel than landing next to a sub. Helicopters provided a more flexible and effective in higher seas rescuce platform. There are still areas where floatplanes are useful, a bush aircraft in Alaska, N. Minnesota, Canada. Firefighting in remote areas with access to lakes. That kind of stuff.

makes sense. Too bad; they're neat.

...still, the US, especially the Navy still uses props (P-3, C-2, E-2C, C-130) and that Beriev jet is neat; it's got a 2000 mile range -- Russia just ordered another one, and supposedly Chine, Greece, and some other countries are interested in them. It would just be interesting to see what you could do if you were designing a fully modern flying boat, from the ground up...

Now THAT is a seaplane. (I assume they would be designed to land at sea.)

yes!

There are just too many of the things mentioned in the original post that are done better with regular ships and planes to make seaplanes\flying boats feasible. This is why the US Navy got rid of their last ones (the Marlins) in the 60's. One of the major problems with flying boats is that you cannot land them everywhere and anytime, they are pretty much limited by the current sea state (how rough the water is). There are plenty of stories of flying boats landing at a rescue site then not being able to take off again because the water was too rough, or the plane being too heavy with rescuee's, it's much better to drop a life raft, then send in a helocopter or ship thats in the area. And the waters around most major cities are just too congested with small boats and ships. Resupply ships by air, that's so funny. We in the US Navy will just keep our supply ships, thank you very much! But really, we already do enough resupply by air, by either COD aircraft, helocopter, or in the case I was involved with, droping by parachute from a P-3, to a Frigate that was in the middle of nowhere, some badly needed repair parts. You could justify the need for a seaplane\flying boat, but you couldn't justify the cost of maintaining a squadon or two of them.

One of my standard stories is about the air force colonel, who shall remain nameless that was transported by a Catalina (one of the best flying boats ever build) from Copenhagen to Aalborg. Now the Cat wasn't the fastest plane, so relieve boredom he resorted to liquid refreshments, and with the time it took, a lot of liquid refreshments. So when they landed in Aalborg he had plainly forgotten it was a seaplane as he stepped off. That is one of the reasons airforces have killed the seaplane!

The only role these days that the flying boat/amphibian is the best at is as a firefighting waterbomber. They can reload with water by skimming low over a lake and lowering a probe. After just a few seconds, they are fully loaded and ready to drop.

In modewrn warfare... they're VERY vulnerable! Sea state dictates where they can operate from... or land on. Arming and refueling are a nightmare if the aircraft is afloat... and if it were an amphibian... it may as well be land based. Jet engines are very succeptable to salt water corrosion... and injestion of sea water. The sea only complicates the corrosion control problem. Other than small harbor areas with little room for an airfield... the small civilan amphibs are good for little more than air taxis and sightseeing. As a weapons or recon platform... no way. We experimented with those (Sea Master and Sea Dart) back in the '50's... got some good aircrews killed in the R&D process.