In reply to Brown:
Yes, the 'estimate' that works for a few days is approx 12h 25 mins between tides. But there is a lot of variation depending on the moon phase. Its goes out pretty quick though .
The android apps I use are.
Absolute tides (paid for subs about £4 per annum)
Tide Prediction. (free, but fewer locales to choose from, but will do next year quite happily )
I'm amazed how good my casio watch is, but it's only dividing the tide into about 6 segments, so ~2 hours each. That works on lat/long, moon phase and lunar-tidal interval (time from moon over head to high tide) ...