Thank you Christine.
I found this :
As an adjective preceding the noun it modifies, underwater is always one word. Some writers use underwater as a predicate adjective (e.g., his head was underwater), and others use the two-word under water (e.g., his head was under water). There is no rule for this, and there is little consistency even within publications. The rule of thumb is that underwater is an adjective and under water an adverbial phrase, but this doesn’t seem to guide practical usage.
I'll go with Under Water then I wonder how Mika writes it