Has anybody grown water bamboos completely submersed? Do they require CO2
and fertilisers? Thanks.

I've a lot of experience with bamboo..... I sell it every day and I've
set up many displays with it... To be honest, my experience has shown
me numerous times it really does not do well underwater, even in the
absolute healthiest, most well-monitored planted tank. It will
probably hang around for a few weeks, languishing and getting paler and
more overgrown with algae... leaves starting to rot... then it will
slowly die. It tends to do best above the water or as a potted plant
in a non-sunny window (doesn't like direct sun).

I'd keep at least half the entire stalk above water... that's probably
the only way that it will be allowed to flourish.... if any leaves are
left below the water, they will probably rot.