You are wasting your time and resources by this habit; it does not help the living or the dead. The dead are not going to benefit from decorating their graves with flowers or water poured on them.

You should spend time and resources in ways beneficial to the living; if you want to benefit the dead, you can give the money to charity on their behalf. The dead do not need physical nourishment or drinks.

As for adhan, its function and purpose are to inform the living of the arrival of time for Salah for them to get ready to join the congregation. Since the people in the graveyard do not pray why should you call adhan there? If it had been beneficial, then the pious predecessor would have done it.

There is no precedent for doing so in the life of the Prophet and the pious predecessors. So, I would urge you to stop this innovation; if you want to visit the graveyard, you should do so and greet them and offer prayers for them.

Aishah, the beloved wife of the Prophet (peace be upon him), asked him: If I were to visit the grave, what should I say: He replied, “Greetings of peace to you, O the believers residing in these dwellings; sooner or later we would also join you. May Allah forgive and you all.”

You may also read Al-Fatihah and offer a prayer for them.

These are the etiquettes of visiting graveyards. So, you may do well to observe them and stay away from all other innovations. The Prophet (peace be upon him) said, “Whoever innovates any practice in this religion of ours –  that is not sanctioned by us – it ought to be rejected.”

Almighty Allah knows best.