When using a tridents riptide attack, anything you hit should apply the tridents damage but when the trident is put in the offhand it applies the damage from whatever you are holding in your main hand.

There are 2 ways to show this, The first way is:
1. Put trident with riptide 3 in offhand
2. Hold nothing in your other hand 
3. Use riptide underwater at a fish and see that the fish takes 3 riptide attacks compared to using the trident in your main hand which only takes 1 riptide attack

The second way is:
1.Put trident with riptide 3 in offhand
2. Hold a sword with fire aspect 2 and looting 3
3. Use a riptide attack at a fish underwater and see that the fish dropped cooked fish instead of raw fish from the riptide attack