First of all, while 21M litres of water you can also split into very very small droplets (of water), it's still 21m litres.
Then, while 21M litres of water you can split into much more very small droplets, with Bitcoin you cannot do the same just like that.

Now, Bitcoin. Indeed, the 21M Bitcoin you can see as 2 100 000 000 000 000 satoshi, but that's still 21M bitcoin.
Now, the problem is that without changes deep in the code, changes which arguably will make it not be the "original bitcoin" anymore (!), we cannot go "deeper" than the satoshi unit, while with water you can go as low as the H2O molecule (much lower than the visible droplet).