Z240 performs just a mono flash, it cannot produce FP flicker 40 khz. Camera shutter window must be fully open at the moment of mono flash, to get uniform exposure along the shot. Nikon mechanical shutter fully opens window maximum at 1/250, not faster. That is why 1/250 is official "sync speed". If user sets 1/320 ... etc., the window never fully opens, it looks like an open strip going along the shot (rear curtain becomes visible on the picture), and it needs FP flicker long flash then.
So, shooting by Nikon with underwater strobes, user must set only up to 1/250 speeds, to get uniformly lighted picture. Yes, 1/320 is threshold value, dark strip is slightly visible (or almost invisible) on some Nikons, but anyway, it is not recommended for common case.