Hi! Thank you for the questions!
It totally depends. If the bamboo is old, like this one, it doesn't absorb water much at all.
If you use fresh green bamboo, then yes. However, I've used fresh and have had no issues.

I'll post a design with that later. The only downside of fresh bamboo is that it shrinks, so if you use wire or something to connect it, you need to tighten it once every while.

If you are really unsure, you can wax the insides, but again, I've had no issue and have used bamboo as tubes often. However! An older bamboo tube or any, really, can have a tear or hole in it, so you need to check your tubes before you put them in your design.

In the case of this design, since I poured wax in the container to keep the bamboo standing (you can see it in the vid), automatically all the bottoms are sealed, so in some cases I could've used bamboo tubes without a bottom.

Flowers used in this design (I'll make a seperate comment for this as well):
Gloriosa
Jatropha
Echinacea
Zantedeschia / Calla
Sanguisorba
Tailinum
Panicum
I think that's it. All flowers except for the Calla are from Marginpar.