More info about the Marangu Route
Bottled water is only sold at the Marangu Huts. The reason that it typically isn't offered on other routes is that porters would have to carry it, and plastic bottles are not allowed in the park.
The Marangu route is a good route to consider during the rainy season. It is the only route that offers accommodation. Therefore you can be sure that you will be able to rest in a dry place under a firm roof.
If you are confident about your physical abilities and you either have considerable experience in high altitudes or you are planning to pre-acclimatize on another mountain, then you may prefer a more challenging and direct summit approach.
In this case, the 6-day Marangu route may be what you are looking for.
If you are very fit, pre-acclimatized or experienced and looking for a more extreme challenge, you could even consider the 5-day Marangu route.
The Marangu Route is one of the shorter routes and is rather steep, so it has poor altitude acclimatization opportunities. Therefore, it also has one of the lowest summit success chances.
The Marangu Route doesn't have a good altitude profile as it offers little time for proper acclimatization. This increases your chance of getting acute mountain sickness.
However, the steep Barranco Wall is included in this route. If you suffer from serious vertigo, you might prefer to avoid this challenging climb.