I cannot find a strong connection with some of the hints, but I think it's something along the lines of:
When I am in the sea, I'm always falsy
In C, NULL is the same as 0, which is the same as false
When you add it, you can finally see me!
NULL doesn't correspond to an ASCII character, but some larger numbers do
When you take a drink with me, people say I'm a fool - but I always drop the l.
'Fool minus the l is 'foo' a common placeholder variable usually initialized to 0 or NULL. Thanks to Lord Farquaad for this insight
Nul is French for zero/null, which might have something to do with 'dropping the l'
But I can't figure out
How this connects to the title