What does it mean that the watchman's words was on the watchtower by day and through the whole night?
The watchman made it very clear that he was vigilant, day and night. (Remember that this is not an actual watchman, but a visionary experience; therefore, being on duty day and night is possible). The watchman did not miss anything, so there could be no doubt about his message.
8 Puis elle s'écria, comme un lion: Seigneur, je me tiens sur la tour toute la journée, Et je suis à mon poste toutes les nuits;