Saildrone Forecast subscriptions are managed through Apple iTunes (in-app purchase).
To cancel, you must turn off the app's auto-renewing option in iTunes, or Apple will continue billing you. Only the Apple account holder can do this.
To access your iTunes subscriptions, the iOS app, click the Settings wheel in the upper right corner and then click Manage or Sync Your Subscription, and then click Change or Cancel Your Subscription.
If the device that you're reading this page on is signed in to your Apple iTunes account, the following link will take you directly to manage your purchases and renewals:
(For additional instructions: https://support.apple.com/en-us/HT202039)