I would do it with xps insulation, working from top of the floor. Its such easy to cut, clean job and finishing using pu foam(the botle stuff) it will be air tighter than any other solution. Would remove any other old/previous insulation. Easy to make flush with existing floor structural beams, so no change inside the house.
If using rockwool You need to build "carrier" planking to keep it in place(not needed with XPS), that need to be done from the underside. Then You need breathable wind stopper material beneath the rockwool if done properly.
Guess 100 mm would be a great improvement so for an 100m2 floor, you need almost
10m3.....(less the beams), might hurt for the transport cost, don't know would rockwool be much cheaper though.
Know that in some houses insulation improved by 2K PU foam, but defo not a DIY job, very risky business, would not be my choice in any case.
fun on a boat is inversely proportional to size...sort of anyway