Yes, water level can be high at the moment when photo is taken, and low on the day after or before, or vice versa. and so changing the coastline based on the latest aerophoto has no point.

For the most part Estonian coastline in OSM appears to be imported from Land Board datasets. Land Board's coastline data is based on long-term means of the elevation data. This is the coastline you see on the basic map and other maps that are derived from ETAK data. I think it's practical to stick to this, and update if the Land Board updates their coastline data.

This high tide guideline really has no use in our case as tides have almost no effect on water levels in Estonia.