Hi Holly
Its not always possible to provide "cheap" and "easy" way when fairly difficult substrates are presented for tiling particularly as the most important thing is to provide a "succesful" result Failure can give a bad example to other often influential people who will then find alternative finishes other than tiles.
If the boards are in poor condition either remove them and fit sheet marine grade plywood or overlay them with the same, minimum 18mm thick, screw fixed down the joists (floor boards) at 250mm centres. Fix ceramic or porcelain to the former in Keraquick incorporating Latex Plus or S2 Ultraflex Quick or to the overlay in Keraquick i.e. no admix.
If the floor boards are in good condition then screw fix each of them to the joists and fix tiling in Keraquick incorporating Latex Plus OR S2 Ultraflex Quick
use Ultracolor Plus grout, 26 colours and no admix required
dock