Bamboo flooring or tongue & groove wood flooring in bathroom?

water will penetrate all wood flooring eventually. Either you use a bathroom specific product or you put ceramic tiles on cement boarding NOT ply, its also susceptible to water as well.

If you want ceramic tiles then overboard the existing T&G with Hardibacker cement board. Then fit your tiles with a good quality flexible adhesive and grout(Not DIY rubish from Wickes or B&Q). A better alternative is to overboard with 6mm wbp plywood and fit Amtico or Karndean vinyl tiles. The benefits are warmer underfoot, less slippy and will not crack or delaminate. Your Builder is not entirely wrong. There are many ways to achieve a long lasting and stable installation. You could also introduce a decoupling membrane that will give further security. Yes you can varnish the existing boards. It won't last forever but that's your choice.

hello the best flooring for a bathroom would be an lvt floor' as it's benefits in so many ways over traditional tiles or bamboo wood engineered floors if you like the look of tile or wood but want a easy clean low maintenance that meets the demands of everyday life it's Waterproof drawable pet friendly and best of all warm underfoot

Sanding and revarnishing the existing floor is definitely an option. As long as there's a suitable varnish or oil applied and maintained, I don't see there being any issues

The best flooring for your bathroom would be LVP luxury vinyl plank as it is 100% waterproof. There are many colours and grains available to suit your style so you’d have no issue finding something you like. It can also come with an integrated underlay which will cancel out your need to purchase the underlay separately. This flooring is soft underfoot and gives a warm feel.