How slippery is algae?

[RJ Dalton 89]

Just what it says. Would you count algae coating a wet surface as slightly or severely slippery? I'm honestly surprised it's never come up before in all my years of gaming, but this is the first time. The enemies are in the middle of a raised platform surrounded by a 40-foot pool that is two feet deep with algea coating much of the ground beneath it. They are relying on the players being hampered by the water and slippery ground to keep them from getting to them immediately, giving their spell caster a chance to get off a spell before the players can get close enough to engage them.
My figuring is that the two feet of water makes them move at half speed, but an acrobatics check could allow them to move at full speed with a risk of slipping and falling because of the slippery ground. Depending on how slippery we'd consider algae, the DC is either 12 or 15. A very slight difference, but enough that it could matters, especially to characters without ranks in acrobatics.

[bhampton]

I'd say that's totally GM call. If there wasn't a lot of algae, I'd go with 12, if the rocks were covered I'd say 15.

[Haladir]

Having had to walk across flat rocks covered with algae in real life, I'd set the DC somewhere between 15 and 18!

[The Black Bard]

+1 to Haladir. Algae on a flat rock is easily equal to ice, perhaps higher, as you can frequently disturb the algae enough that whats on your foot breaks free of whats on the rock and you now have two slick surfaces sliding against each other, instead of rough on slick.

Thankfully my youth was spent before the advent of modern cellphones and youtube, since I lived next to a creek. So many dumps in the drink because of algae-under-foot.

[Meirril]

How high do you want it to be?

If the rocks are rough and divited its a lot different than smooth rocks. If the alge is like a thin greasy film over a thicker layer of alge that also slips right off the rocks its a lot slippier than moss clinging to a rock that actually provides a bit more traction than just a smooth rock.

But seriously, this is a game. You decide what you want the DC to be appropriate to the expected level and who you expect to pass this.

[merpius]

Ever been to tide pools? Crap, those rocks are covered in highly-traction-giving barnacles and are, somehow, still easily as slippery as ice from the algae and seaweed. Almost a certainty that you are going to get a significant abrasion (from falling on the aforementioned barnacles).

For a DC; I'd totally give it the Ice DC unless there was hardly any algae at all, or the algae was dry.

[Thebazilly]

My last real-life encounter with algae was climbing out of a river bank in the dark after an afternoon of floating in an inner tube. I would agree that using the rules for ice would be appropriate.

Since I wasn't familiar with those off the top of my head, I looked them up here. Interestingly, this fits well with your idea for speed penalties due to the water.

Of course, you could increase the DC if needed. Instead of a smooth bottom, the pool could have large, rounded river rocks which provide poor footing and shift under the characters' weight. Or barnacles, as merpius suggested, or perhaps the water is murky and the characters have to feel out every step on the uneven bottom.