The book doesn't explicitly state whether or not the spectators could see anything, but it heavily implies the second and third tasks cannot be seen.
After the second task, Dumbledore gets a report from a merperson on what happened during the task, which implies he himself didn't know. (emphasis is mine).
Dumbledore was crouching at the water’s edge, deep in conversation
with what seemed to be the chief merperson, a particularly
wild and ferocious-looking female. He was making the same sort of
screechy noises that the merpeople made when they were above water;
clearly, Dumbledore could speak Mermish. Finally he straightened
up, turned to his fellow judges, and said, “A conference before
we give the marks, I think.” . . . “Ladies and gentlemen, we have reached our decision. Merchieftainess Murcus has told us exactly what happened at the bottom of
During the third task, a lot of terrible things happen in the maze, including an unforgivable curse and the disappearance of two students. If the tasks were visible, why didn't Dumbledore do anything when Krum was using the cruciatus curse on Cedric? Had Harry not stepped in, Krum would have killed Cedric while under the imperious curse.
Again, it is never stated in the book whether or not the tasks were visible, but the evidence leans towards no for the second and third task.