Tidal Adjustments based on high water at Shell Island
| Location | Calculation | Time Adjustment |
| Aberdovey | Subtract | 2 Minutes |
| Aberystwyth | Subtract | 28 Minutes |
| Abersoch | Subtract | 15 Minutes |
| Bardsey Island | Subtract | 16 Minutes |
| Barmouth | Add | 5 Minutes |
| Criccieth | Subtract | 2 Minutes |
| Cardigan | Subtract | 39 Minutes |
| Fishguard | Subtract | 39 Minutes |
| Milford Haven | Subtract | 2 Hours 9 Minutes |
| Pothmadog | Add | 17 Minutes |
| Pwllheli | Subtract | 5 Minutes |