finish for bathroom wood walls

Dear all
I just wanted some advice on what finish product to use on our red ironbark bathroom walls. We need something as waterproof/water resistant as possible, which expands easily and does not go yellow. Hopefully it would be easy to apply and not need to be redone more than several years apart, if at all.
We have been given various advice, including Bona water based polyurethane, and Sikkens Cetol TS interior (she said only one coat to allow it to breathe, which sounds strange – I did not think we would want it to breathe in the bathroom).
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Treatex is another option although it contains linseed oil and I suppose that would be a mould risk.
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Does anyone have any feedback on these options or have any other suggestions?
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Any advice greatly appreciated.
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Thanks very much
Sandstonehouse