contracts and payments for crew

[scuba0_1]

I have read so many threads of i did this and didn't get paid....
I just sold my power boat and the buyer wanted a water test after every thing checked out on land. I had him sign a contract and leave a deposit assuming that the water test was fine he was to buy the boat or lose the deposit.
Those of you who move boats or crew with pay, is it normal to get paid after the trip or as you get on board. If your going from point A to B and I'm on board i would want to get paid if there boat made it or not and don't you sign a contract before you step on the boat?