We knew that the floor was rotten under the sink and in the doorway. The existing floor also had sunk at the ends. Overall it was feeling quite spongy.
The two wood cross-members supporting the floor were also rotten.
In the end, having replaced so much rot elsewhere, it seemed a shame to not replace the entire floor.
Although we had pulled the floor lockers/bunks out, the full height wardrobe was still in place. That was going to make replacing the floor section there tricky.
In the end, we had to cut the old floor out in sections, and then replace it with new plywood in the same way.
With a bit of muscle, and not a little swearing, we were able to slide new plywood in under the wardrobe without removing it.
The front and rear beams were replaced with treated timber at the same time.
Most of the joins were around the sink/wardrobe area. We stiffened the floor underneath in these areas with another section of 12mm marine plywood, glue and screws.