The reason for the corrosion is relatively unimportant beyond that it is "high tech" in origin, in contrast to the society in my setting (It could be a complex mix of industrial pollutants or plastic-eating bacteria gone out of control, I'm taking suggestions as well). For reference, the setting for this world is a primitive society (secretly the post apocalyptic remnants of humanity) living on floating islands over a shallow ocean. The ocean is a source of resources but also danger, and the corrosive property of the water emphasizes that.
As for how corrosive the water is, perhaps something like a human spending 5 minutes in the water is sufficient to result in 1st degree burns all over the body.
But as I said, all that is not the focus of the question. What I would like is some suggestions on how life would look like in this kind of ocean as I think I lack the biological and ecological knowledge to create a compelling depiction. This is perhaps 10000 years after the point where the oceans started turning corrosive, so not too much time for evolution to happen. I'm thinking the changes would be more in the relative distribution of different types of aquatic life rather than the forms of the species themselves.