I thought it was free through NOAA, but not sure how that is handled commercially.
I know you can get free tide data with a personal api key on wunderground, but that may be cumbersome to have users signing up for it and entering their key in the app.
If no one comes up with a better solution, I would suggest a paid plugin/addon option. I would be happy to pay for that or even donate for the app as it is. I still can't believe this app is free so maybe you don't ever want to make it commercial?
Something to consider though...
In 2010, 123.3 million people, or 39 percent of the nation’s population lived in counties directly on the shoreline.
Tide data could really help a lot of people. Wish I could be more helpful with ideas to implement it.