As it transpires, some of the barrels are already open! None of the sealed barrels can be opened easily in any way that Sable can determine unless she's very familiar with how barrels work, but there are in fact a number of already opened barrels on this deck. 

Most are empty, some smelling of old fish or salt in a very different manner than the sea, and none in fact seem to have gunpowder in them, but one is half-full of a very dark substance that looks viscous and extremely sticky. 

Sable pauses at the barrel of sticky, tilting her head. "Huh."

Sticky stuff? On a ship? Maybe some kind of... finish or sealant or something? What would they use...

"Tar!"

Tar is made from oil and it burns she can make torches! Assuming torches can just be a wad of rags on the end of a stick and dipped in tar. Maybe soaked? She'll figure it out.

Okay, back up the stairs, go get a stick, then hurry back down and tear or cut a big strip off one of the most frayed bits of hanging sailcloth.

Now does she tie and dip or dip then tie? If she dips first then there's tar — fuel — all through the wrap, but that'll make tying messier? She doesn't wanna get tar on her hands. Maybe if she dips the middle and leaves the ends out she can just hold onto those? That's worth a try.

Into the tar goes the strip, then it gets wrapped around the end of the stick, back and forth, starting at the middle. Finally she ties it off, and holds it over the barrel for a bit to drip away any excess tar.

The tar is very sticky, and she ends up with it on her hands and anywhere else she puts her hands afterwards, unless she spends some effort to try and clean them, but after all this effort she ends up with something that looks like a torch, and will probably work as a torch. 

Ick. Gross. She has got to find a way to do this without getting sticky next time. For now she'll just use another scrap of sailcloth and maybe the water in the shallows to wash her hands, vigilant against any sign of tentacles while she does. 

The shallows, at least when she visits them this time, are tentacle-free. 

Good. Once she's got her hands clean, she'll light her torch and explore the first lower deck more thoroughly, and then descend further below to do the same.