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Following the other comments, in the long run you probably can't. Stone of some kind is a better bet. The more varnish/sealer you apply the more pressure there is from the moisture in the wood to escape, moisture vapour permeable lacquer might work if it's available. Then you have the 5-10 year maintenance cycle. If you don't mind u/v bleaching and drying the wood every time it gets used then it might last even with wax. If you put tenants in I'd give it about a year before it was rotting.
I can't understand why they would do it all in knotty pine either, the resin leaching from the knots would destroy any treatment. Climatic conditions would affect your choices though, this could be a great idea in a Swedish summer lake house.
How would you stop the wood surround on this bath from warping? Paging @Airhead :)