all the yachts I spoke to, basically they employ people for the particular job and help out in other jobs onboard. They wanted to employ me as chief eng. and poss do some flying but I wanted to be employed as pilot and help out as engineer. Pay for engineers was from about £2k/ €3k to start and up to about £6k/€9k per month. It was tax free and had health/life ins/small travel allowance + yearly bonus up to 20%+charter tips depending on vessel. I only ever worked as engineer onboard a large yacht but on one occasion I made $1k in one week from tips. (famous film start chartered the yacht).
I know there is at least one pilot who frequents Rotorheads who flies off yachts and I guess he prob makes about 100k+ year. Again, depends who you work for but all salaries are pretty much negotiable to a point.