If you have an iPhone, live in the UK, and go sea kayaking, give yourself a little Christmas present in the form of this app.
Boatie is particularly handy when away from home for a few days.
It's most useful for weather forecasts, with access to Met Office Inshore Waters and synoptic charts. For both you need a mobile phone signal.
It will also provide tidal data, but again a signal is needed.
The World Tides App had all the tidal information inside the app, so no signal was needed. However, it no longer seems to be available. I hope a 2010 version will be released.
My next most useful weather app is WeatherPro. A combination of these three works well for me.