How can you say that a guide that is an owner/operator does not work for wages? They not only work for wages, they have to pay for the boat, bait, equipment, dockage, insurance and any other cost they incur. What's left they pay taxes on and that becomes their wages.
Any guide offers a service, if one is happy/unhappy with the service rendered it is up to that individual to tip or not to tip.
IMHO if I was happy with the total experience the least I would tip would be 10% and might go as high as 20%
EDIT ADD:
This same kind of debate comes up about tipping bush pilots.