Does anyone know how well the OM-1 mark II does white balance in shallow water. Looks like it has a pre set white balance for it ?

Perhaps you could explain why this is important in your case?

I take photos with the OM-1 and OM-1 II, but white balance is not important to me as I only shoot in RAW.

Jens is right. If you're shooting RAW then White Balance is simply a metadata value that doesn't have any impact on the image except in its preview on the camera LCD screen. Once you have the RAW files in the computer (Lightroom etc) the image will display with the original WB value but it's not baked in and you can freely change it.

A common and useful technique for underwater shooters is to set the WB to the same temp as your strobes - this at least gives you a consistent preview and baseline for comparison and review, but then you're free to warm up or cool down shots as you please.

But if you are shooting JPEG, perhaps because you want to use the photos straight from the camera without processing, then you're trading off quality and WB flexibility for speed and efficiency. If you're shooting in shallow water there can be lots of variable sunlight and the colour temp can change quite drastically from shot to shot. So it's not a case of how 'well' any camera does WB. They all do it the same. It's a matter of either either letting the auto-WB do its thing (OM auto WB is excellent but in variable shallow water all cameras will struggle). Or manually changing the WB depending on the colour of the water (Blue or green) and whether you are facing upwards to the surface or downwards away from the sunlight. In all practical terms, it's very clumsy to be changing WB for each shot. So best to find a consistent fixed WB temp that matches the water and type of shooting. or else go Auto and hope for the best. Most of the time it will work.

JPEG will allow for some degree of colour temp adjustment later but this does bring us back to RAW and why really, RAW is the only way to go.

HTH

Edited 1 hour ago1 hr by MikeJonesDive