What a difference a decade makes! An edition ago I couldn’t have told you what a grindylow was. Add some China Miéville and Harry Potter into the mix, and now it’s practically a household name.
Despite their low CR, two things make grindylows scary: First, because of that low CR, PCs are likely to encounter them before they have access to regular or reliable water breathing effects—making fighting them anywhere near their own turf a dangerous proposition. Second, look at that organizational chart. Grindylows can scale up fast, both in levels and sheer numbers. So a party should be encouraged to get in over their heads—literally and figuratively. After killing one or two of the creatures, PCs are likely to get cocky…only to find themselves surrounding by 40 of the savage, needle-toothed beasts, along with all their pets, leaders, shamans, and mutant cousins.
The shark-like, bloodthirsty adaros are hardly sympathetic figures. So when a mother adaro and her child wash up on shore seeking sanctuary from a warband of grindylows, a seaside village has a hard moral choice to make.
When raw octopus replaces the traditional fried squid as a delicacy in Portuwar, their fishermen leap to take advantage of the new market. This angers the nearby grindylows, who begin swarming upon solitary ships en masse, leaving torn nets and skewered men in their wake. Soon even Portuwar’s piers are not safe, especially at night.
A river is locally known to be haunted. In truth, it is the domain of a grindylow matriarch whose monstrous spawn are particularly bloodthirsty due to some unknown force of corruption. The local water nagas sometimes war with the tribe; the local nixies prefer to avoid them, but are not adverse to luring mortals who have offended them into the grindylows’ clutches.
—Pathfinder Bestiary 2 148
Since we’re talking aquatic creatures, now is perhaps the time I should mention that I enjoyed Mike Shel’s Isles of the Shackles a lot. Look for the usual nice omnibus of locations and adventure ideas, plus a particularly hefty does of monsters, which struck a nice balance between general (duppy, larabay), setting-specific (Aashaq’s wyvern, gholdako), and useful NPCs (pirates, jinx eater).
I’m also digging the Skull & Shackles Adventure Path—my current Aubrey–Maturin obsession having a lot to do with that—and so you can probably guess how much I dug the look at Golarion’s ocean races in Pathfinder Adventure Path 56.