Hi Elinor,
Any good flooring contractor will be able to repair your bathroom. I'm unsure if plumbers do the structural work you may need on the floor but I know most good flooring contractors do on a regular basis. Depending on the flooring you currently have there you may want to look for that specific contractor. If you have tile find a tile contractor, if you have vinyl find someone that does that, etc. Each specific flooring has different specifications for the subflooring beneath it. Tile needs a different subfloor than vinyl or carpet.
Hope that helps.
Roger
Tile and Stone Contractor
Fort Collins, CO
TileArtCenter.com