Well there sort of the same I guess, in the sense that both take electricty and make sound, but as Blueroom says, I horn is more of a buzzer with a single tone. I horn will make noise by simply connecting a direct current to it, a speaker needs a circuit to generate a tone in order for it to make a sound.
I don't know that a motorcycle horn is waterproof...... Weather resistant maybe, but certinly not made to be used under water.....
. . .working at a freq. designed to cut through traffic noise.
Cars have dual horns with a high and low tone, the high to cut through noise and the low to carry distances.
It takes much more current to drive the horn, which is likely to be rated for 12V and quite a few A, than the small amount required to drive the speaker.