Tides and Waterline
The tides should be tracked via the National Tidal and Sea Level Facility, which allows access to UK National Tide Gauge Network data.
The closest gauge to the site is probably Newhaven. https://www.ntslf.org/data/realtime?port=Newhaven allows access to the tide gauge itself. Some additional info on the gauge is here. Note that you will have to subtract 3.52m from the tide gauge levels to convert to OD (ordinance datum), commonly known as mean sea level or map zero.
For February 2nd, a low tide is expected at 4:09 in Newhaven. This means the tide level should be realitively stable from 3:30pm onwards.
The tides may be lower than the waterline, as water tends to run up the beach; with the waves and all, the waterline may not be easy to determine. I suggest setting a procedure to determine the location of the waterline so that it will be carried out consistently in both locations. I expect that the tripods would need to be set in the surf for the experiment to be carried out as indicated, which will detriment the stability of the platforms, and require gum boots on part of the experimenters, so it may make sense to specify a distance from the waterline as the location.
Some people can tell time by the sun, but I could never make out the numbers.