I'm planning to build a sunken bath/wet room type shower. The plan is to raise the floor of the bathroom by 18" and then build a bath with its bottom just above the original floor level (to allow for drainage). What is the best way to build, waterproof and tile this custom tub. It will be box shaped with no curves. Basically like a sunken wetroom shower.
I once built a bath floor with white cement, and then covered it with sikatop-107 seal. it was an outdoor shower but it will fit also for an indoor one. here is a link to an article i wrote about it in my website. sorry, it's in Hebrew - but you still can look at the photos :)
you can build a structure out of salvage materials, plaster a thick layer of cement on it, and finish it with the sikatop-107 seal.
You can buy wet room kits that comes with sealant coatings, corner tapes etc. Perform a quick search on YouTube and there are a number of How To videos for installing a wet room shower. Obviously you are not doing a shower, but the process would be similar.
Once the surfaces are coated with the sealant you can then tile straight over the top.
Having said that, most baths do not actually have that much water outside it, bar the occasional splashes etc, therefore these can easily be dried up before it causes any damage and most tiled floors etc will provide enough protection for the short term. If you envisage that the water will be constantly settling on the tile floor, then the wet room sealants would be the best approach. Naturally of course you will want to use a good mold resistant silicone sealant around the bath tub edges also.