At first I switched mayo for a little olive oil. While tasty, it can be a bit dry. I use Chunk Light in Water, and when I open the can to drain it, I don't drain all the water out. I add my seasonings and mix with a fork, and it's just as moist and easy to spread as if using mayo!
If you find you left in too much water, just throw in a tablespoon of ground flax meal, or crumble a Saltine cracker or some oyster crackers to make it less watery.