Polymorph target (sea monster) with con dmg or good combat abilities?

[Dominion]

I'm a lvl9 sorc and can cast under water (Eschew Materials, Silent Casting).

We are on an island and a lot of ships with a lot of hungry mouths need food.

My idea was to catch a beast from the island to have sth that wiggles, kill a human to have some blood, put a chain around them and drag them behind a ship until something bites.
That something may or may not destroy the ship, but that's secondary.

I would then jump into the water with my familiar. Share spells mage armor + haste + polymorph.
What would the ideal polymorph target be?

So far my best bet was Darktentacles (https://www.realmshelps.net/monsters/block/Darktentacles) because of its feats and 12 slams (13 if hasted times 2 bc my familiar is also morphed = 26 attacks per round), but I would very much prefer something with poison.
Sth like con dmg to guarantee a kill no matter what bites.

I have a necklace of waterbreathing so creatures of my size with a neck could work too, but I think I need to stick to the target (eg have swim speed).
Our DM has a habit of letting monsters run away.

Thanks in advance :)