Impress your friends (and strangers) by knowing at the drop of a hat whether the tide is coming in or going out. Anaconda are having a sale on this watch, which I could not find cheaper. It's for club members, but membership is free. Comes in three colours:
No delivery, but stores seem to have some for in-store shopping and free click and collect.
Edit: add code BIRTHDAY10 for $10 off (thanks @tdw!)
2nd Edit: seems like stock is running out, but some stores still have it.
This can detect tides?