Seems impossible to beat the AI. I have played many times and AI is very overpowered. The few fish I do catch are small because there is not enough rounds to buy better cards. Maybe my opinion but the game seems unbalanced in Solo play.
I would agree that as the original rules (no official variants) the game is very challenging, but not impossible (I have won 1 out of 9 games )
One question for you, do you have any expansions? If you have Rods and Boats, having the boat and rod card in your starting deck helps a ton as you can be effective more in the early game.