Sorry, I'm too lazy to take a screenshot... because my game is on another computer... I don't know what type of seaplane the poster is talking about? Helicopter? Or a regular water glider?
Let's talk about the water helicopter first. On the bridge north of San Francisco, there is a small town southeast of the bridge. You can find it on the map. . This town (it's quite luxurious) on the south end of town) is the boat driving school. You must have already learned it. There is a helipad at the driving school, and sometimes there is an ordinary Bell helicopter. sometimes it's a water fly helicopter with a machine gun. There is a hill across the sea to the east of this tarmac. On the hill is a broken ruin (very small), like an altar. There is an ordinary flying boat next to it.
And south of Hoover Dam, aren't there two rows of things that look like power plants? There is a helicopterritime and a seaplane on the water at the south end of the power station? on the right~the two are together.
That's about it. There are many places where seaplanes are available, but they are not easy to describe~