Would make for a nice little firebase to be a cantrip turret from. You'd probably have to do it at least once anyway, so you could see what they looked like, so you could wildshape into one later if you wanted to (an often overlooked feature of Conjure Animals).
Hmmmmm..... In theory, a Giant Octopus Druid could use a Quarterstaff if it knew how (tentacles are about as good as hands at many things). So while you might not be able attack at your reach with it, the second part of the Polearm Master feat should apply if you had it, because you could certainly wield the Quarterstaff while attacking normally. You've got 8 tentacles and only 1 attack, so holding a stick while hitting them with tentacles isn't hard.
Tada! 15' reach anti-movement reaction attacks.
Giant Octopi also have weird synergies as an AniMonk. Flurry of Blows, Way of the Open Hand and Fists of the Firesnake all do pretty cool things when wildshaped into one. Extra reach attacks, knockback or proning, or 25' reach flaming restraining attacks. And now that you don't have to be hella slow in movement, perhaps worthwhile considering.
Of course, a Longstridered, Barkskinned Killer Whale is a tank and a half with flurry of blows. It might be slow, but when it gets there, kaboom! Not bad at lvl 11 anyway (15d6+12 damage potential, +6 to-hit, 16AC, 90 HP, 10' movement on land, 120' Blindsight).
It gets better if the Monk's Unarmoured Movement transfers over to Wildshape forms. Did they ever clear that up, or is it still standard "DM decides" (which is always in effect anyway)? Because it doesn't look like much until the weirdness of a super-turbo-digger 30' burrow G.Badger or 30' land movement G.Octopus turns up..
Longstridered, Killer Whale AniMonk of Doom! (20' movement of doom, anyway. DM decides "No. That's Stupid." So you get someone to cast Fly on you. Then it's Stupid.)
Hell, does Unarmoured Defense transfer? Say goodbye to Barkskin if it does.
Don't even ask about Wood Elf movement, no matter how physically able a G.Octopus is to have its base speed increased to 35'. That one's a no, and I'm not even your DM.