Camera rates reflected Pre-flash to determine main flash duration, for TTL automatic lighting control.
Pre-flashes are emitted by external uw strobe using TTL-Converter, or by pop-up camera flash. In communication protocol of camera and flash, camera sends special commands regarding Pre-flash beginning and necessary Pre-flash duration.
In common case, Nikon sends 2 Pre-flashes of different duration, Canon sends 1 Pre-flash of fixed duration.
Yes, user can go to Manual mode (without Pre-flashes) at camera side, switch off Pre-flash camera commands for pop-up flash on both Nikon and Canon systems, very easy, - set M mode in pop-up flash partition of camera menu. Nikon and Canon, both have the same partition in camera menu for pop-up flash control. You can use it, if you have optical synchronization by camera pop-up flash.
But for external land strobes control (or other TTL devices on HotShoe), only Canon can switch off Pre-flash by camera menu directly, Nikon traditionally does it by external devices. That is why Canon TTL-Converters use direct way, - camera command "M" mode" in menu, but Nikon TTL-Converters use indirect path for switching system to M mode by camera menu (for example "Auto FP", or other command), but final result is the same as in Canon system.
It makes sense to better switch off Pre-flashes from camera side, if you want to shoot in M mode by your underwater system.
Shooting in TTL, you must have Pre-flashes anyway, this is camera standard mode. This is necessary for automatic lighting exposure control.
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