Arrr pirates! Here be some tips from captain MacDougal 'bout the Prizes and which give more often rewards.
Here below you see the pictures of the Sea Fort and the Safe Haven - the tip is that these have higher chances of giving rewards than the Caravel and the Black Corsair (also picture below).
And on top of that, the second tip is that the Forts drop prizes more often than Havens. And Cosairs more often than Caravels.