The spawn conditions for squid are easy to meet.
But utterly evil and annoying.
If you have even one block of water going into a current on all sides, a squid can still spawn there.
The odds of it happening however, are very very low.
Also, as they prefer to spawn in 3's, it MAY be more likely to spawn when there's at least 3, 1x2 water sources. But the utter randomness of the spawning means I have no reliable idea.
Also I've found that they can go against a current if they're facing the right way. Currents don't stop them much.
So... BRB with quick sketch of a possible trap.
Ok, so here's the design.
However. It's so impractical it's not worth it. A quick boat ride would yeild far greater results.
So I banish this idea till squids get a proper spawning xD