Something Fishy this way comes

[cartejos]

I'm looking for some kind of ritual or such that would make a creature more like merfolk (without actually being merfolk)

I'm thinking something along the lines of Dragon born mixed with Shadows over Innsmouth

[Maat Mons]

I believe Savage Species has a ritual for acquiring a template. And Stormwrack has the +0 LA amphibious template.

Alternately, you could have were-barracudas. Or were-giant-octopuses. Or were-sharks. But clearly the were-acudas are best, since they have the best name.

[cartejos]

Looked through and found it, Ritual of Gills in Savage Species, thank you

[ShurikVch]

Check some transforming PrC:
Darkwater Knight ([I]Dragon #314)
Deep Thrall ([I]Dragon #300)
Shoal Servant ([I]Dragon #300)
Thrall of Dagon ([I]Dragon #349)

Also, I'm afraid to be obvious, but did you checked properties of Aboleth's slime?