I have a little trick that you can call 'superstition' if you like, but it does seem to have worked out for me a significantly large number of times...
When dropping an untrained farmer into the ocean for the very first time, I drop him or her right at the top left-hand corner of the sea, move the guiding hand to just below the coconut trees and, not letting go of my mouse, keep the fingertips of the guiding hand pinched together (as though I'd just selected a villager) until the farmer in question emerges from the sea with a fish!
I don't know why this should work, but I seem to have a lot more first-time successes with untrained farmers this way!!!