Hi, sorry to reopen an old thread... I'm also looking into tides for my own watch face. Where I live (QLD Australia) we have nice reliable semi diurnal tides (a high tide every 12h25m). I have created a formula based on this principle, but it relies on me having to calibrate the formula using the time and date of a known previous high tide. As such it's a great feature for me as my tide chart is set for my local beach, but its not a solution for putting into the marketplace as it would be wrong for everyone else, and wouldn't work for areas that have unique tidal periods.
The formula below provides a proportion of tidal period:
63692448720 is the date and time of a previous high tide converted to seconds. 44700 is 12h25m represented in seconds.
It would be great to figure out a way of automatically calibrating the formula based on location or user input. Or i guess another workaround could be to adapt the formula so that you have to sync the watch face at high tide of your local beach then the timer just cycles through a period of 12h25m thereafter.