Both the US and Russia abandoned their large seaplane tests and plans some time ago.

Major issues include weather and corrosion. Other issues include the fact the most efficient aircraft design is not one with a ship hull adaptation for waterborne, takeoffs, landings and taxiing; this alone, given the cost of fuel, would likely make use of large seaplanes prohibitive.

That having been said, there are places, usually not the kind that would have hundreds of passengers arrive or depart at once, where waterborne aircraft are just fine - which is why there are many (generally smaller, up to "twotters" or the like,) in use.