This is one of the best games I have ever played on the NES.
What I enjoyed was how well the game was designed. It wasn't the prettiest game out but it was pretty good for an NES title.
Konami did a heck of a job making this game easy to follow and understand. You collect devices to help you advance the game (boots, keys, a radio and etc...) and make the journey easier. At some points you come across doors in the game that you can enter. When you do, the game changes from a side scroller to an old fashioned 3D room view like from Ultima. Its in these rooms that you use a lot of your items that you have collected to advance the story and solve the puzzles of where the kidnapped mermaid is located.
I loved this game so much that even after finishing it I went back and beat it several more times just to see how much faster I could get it done. too bad I still don't have a working NES because I would definitely pick this game up again.