I agree with limeyangler and wallyboss. But to each their own.
I am a transparency kind of employee and manager. Just be open with your plans. The approach will provide and bring more respect.
If an employee has to hide behind a sick day to go fishing, maybe they really ought to reconsider a few things. If I was aware one of my employees had and demonstrated this kind of attitude, I would reconsider their value, commitment and possible future role and employment status.
Just my opinion. Accept it or not.
All said, I have gone fishing before work and came in later to the office before. I even worked from the boat a couple times all day long while on numerous conference calls. I didn't go out of my way to hide it. I didn't advertise it with everyone either. One time my manager asked me how the bite was out in the water and offered to attend a meeting on my behalf so I could bail on one of my conference calls.