That is a great question with lots of potential answers.
If you’re talking about trout fishing or salmon fishing then look no further then the Morell River in Kings County.
If it’s mackerel you are searching for then perhaps try your luck at any wharf on the south side of PEI. I can speak from experience that the wharf in Souris is a great place to get your lure wet and try your luck at a mackerel.
The other option is Striped Bass fishing which can be caught off of most beaches on the north shore.
I hope this helps you decide on your fishing adventure.
P.S. Please check the PEI Government website to read about the rules and regulations for fishing in PEI.