How does a Tide Watch work?

[DiscoStu]

My girlfriend has just taken up surfing and I was thinking about getting her a surf / tide watch. Do they actually work, and if so , how? I'm looking at the Casio and Timex ones in Argos cos she's not too fussed about the fashion side of it. Would they be good enough to tell what the surf's like at our local beach? I've tried their sites but couldn't find an explanation of how it worked, just a list of features.

[heathfield]

A tidal clock may have 'High Tide' marked at the normal 12 o'clock position, and 'Low Tide' marked at the 6 o'clock position. The single hand on the clock will take 12 hours and 24 minutes for a complete 360� rotation. (This being the average time between consecutive high or low tides).
Once set, the clock is only good for a single location - if you move along the coast, the high and low tides will occur at different times.
Some of the watches available might show a graph of tides, and may also show temperature and barometric pressure, but none will be able to actually tell you if the surf's going to be any good, since this will depend on waves and swell - something the watch can't tell you.
If it's just the tides your gf wants to know about, she could instead just look up a tidal prediction website, like this one