How about this? You can be in the skiff with a GOOD QUALITY digital scale that preferably measures to atleast the nearest ounce. Let each angler weigh up to 10 to allow for culling and base the winnings off each persons top 5. A dead fish will only be worth 1/2 its weight and will automatically be 1 of the 5 for final weight. That should provide for some incentive to keep them alive. Furthermore have each angler have to stay within 2 casts of a pre-chosen cell phone carrier for safety and to make sure the catch gets weighed and released in a timely manner.
Think about it.

Tight lines,
Wade