There are a few spells that go through it, which also applies to sivir's spell shield. Mainly Dot abilities though
w
ult
e
w
w
ult (this one is super complex, using a qss will drop the fish, but you'll still be heavily slowed, spell shields will block the damage, but you'll still be knocked up, and as far as I know, it's impossible to block the fish from reaching it's destination)
fizz q ALSO goes through spell shields, due to it's physical and magical damage components, so you only block one form of damage.
ult
e
w
q is actually impossible to block, trust me, i've tried
all ground targeted aoe abilities that do damage over time aren't blocked, and most multi-hit abilities only block one spell