[One, Two, Middlesboogie]

It's possible that Sligs are both aquatic and arboreal; just as humans once were.
The vast majority of primates are tree-dwelling, and that explains our opposable thumbs; and the fact that we can throw spears and javelins, an action which uses the same muscles as brachiation.
Some human biologists reckon that humans also have a partly aquatic background, because we are one of the few creatures whose nostrils point downwards; a feature which stops us getting water up our noses. And, according to the same people, the reason why we have such weird-looking ears is because all the folds trap air bubbles, and thus stop water going up our ears as well. And we certainly don't have webbed digits (not as adults anyway; in our embryonic states we do in fact possess webbing on the hands and feet... occasionally people are born with the webbing still intact and have to have it surgically removed).

I think it's certainly plausible that Sligs originated as tree creatures, then developed some aquatic skills as well. Personally, though, I think they look more like tree-dwellers; especially since their tails arnd tentacles are prehensile, which would make up for the fact that they only possess two limbs.