I mean… it isn’t like that can’t mean water- but they didn’t specify a monoxide so it could be any oxide of hydrogen including H2O3, Trioxidane- which you would not likely want to swim in unless it had reduced to H2O, dihydrogen monoxide.
In fact, as a general warning and not specifically for or by chemists- peroxides, while usually noted along with superoxides, are a type of oxide. It isn’t precise but it is not incorrect to count H2O2- hydrogen peroxide, as an oxide and so that lake could also be full of hydrogen peroxide- which you probably wouldn’t want to swim in undiluted- or arguably even at anything but extreme dilution. Regardless of convention, in practice warnings and such do not always adhere to strict technical convention so much as some regulatory convention.
In fact, as a general warning and not specifically for or by chemists- peroxides, while usually noted along with superoxides, are a type of oxide. It isn’t precise but it is not incorrect to count H2O2- hydrogen peroxide, as an oxide and so that lake could also be full of hydrogen peroxide- which you probably wouldn’t want to swim in undiluted- or arguably even at anything but extreme dilution. Regardless of convention, in practice warnings and such do not always adhere to strict technical convention so much as some regulatory convention.