You need an outer coating of some kind for the fiber, because immersed in water you still need a steep dielectric constant change from the fiber to the outside to keep the optic signal intact.. Air will do but water is most likely too close to glass. However the fiber seem to come with a coating since it says IP67. The weight of the spool is given in the data sheet with 10kg/25km. So you wind up with 20kg+ weight, which is not much for a shaheed size drone and you loose that weight if you spool it from the drone.