If we must hijack this tread to talk about the interpretation of the USCG definitions, then the rule (5a) above has some fudging areas... It mentions "profit", how would you prove it was a profit or a loss? It mentions voluntary, how do you define voluntary?
I think that defining "voluntary" is pretty easy. If the captain will let you come along regardless of whether you pay anything or not, then it's voluntary. If the rule is, pay or you cannot come, then it is very clearly NOT voluntary. In the latter case payment is a condition of being allowed to come along and as such you are clearly a paying passenger--captain must have a license, and vessel must be inspected.