This page allows selection of a tide station (port) from Apple maps.
Pinch to zoom in or out and scroll to get to the area you are interested in. Tap on the pin to select the tide station and confirm that you want to use it to get tides for. Confirmation will return you to the tides page where the tide details for the selected station will be shown.
The location button shows a list of the 6 nearest tide stations to your device’s location (provided that you have location services switched on and you have allowed the app to get your location). Select one and you will be returned to the tides page where the tide details for the selected station will be shown.
To enable location services, go to Settings > Privacy & Security > Location Services and enable them in general and also specifically for the UK Tides app.
The left arrow button will return you to the Tides page without selecting a port.