Hi Gillian
I did this tour in 2016 and you don't really get wet, bit of spray was about it, I took my crocs for the boats and just some reasonable shoes with a bit of grip for the walks, I recall getting my shoes soaked on the beach at Jaco's Costa Rica watching the sunset as a wave took me by surprise, they took days to dry out and were in fact ruined, there may have been at that one area where we boarded the boat to go to lodge was a little mucky but I can't remember off the top of my head if that was borneo or CR
money i'd say around £300 but you'll need almost as much again for Singapore
all the best any questions just ask
Dave