First of all, were I you, I would pull that toilet and get it out of the way. I would then pull out the tub, and replace the entire bathroom floor with a good grade of 23/32" plywood, with all edges blocked. I would then cover the wood with an SBS peel and stick material, and turn it up the wall about three inches. Your tub looks to be in pretty good condition, and I understand they have a spray on refinishing material nowadays that is pretty good. You would then install floor tile as you wish with cement backer board and thinset or a wet-set.