The hydrographic charts have been coloured for many years now: yellow for
land; white and blue for sea and green for sometimes sea, sometimes land.
These sand, mud or rock areas are avoided by most boat navigators. However, in a
region with a big tidal range (Bristol Channel or Irish Sea) they can be crossed
when the tide level is high enough. Sometimes that saves many miles, sometimes
it is necessary to reach a safe drying anchorage or mooring.
Unfortunately, the Hydrographic Office surveys of such areas are often non-existent or spectacularly out of date. While their paper charts contain information on date of survey, the electronic charts used in plotters do not have that useful information. The Ordnance Survey is not much better - their LW and HW locations are often derived from quite old data (and, annoyingly, it is very hard to find the date of the survey, other than by comparing maps of different dates to look for similarities).
A more recent source is satellite images. Google Earth allows the date to be varied which is helpful. Their images are at varied tidal states, so one is lucky to find a good LW image. Bing also has such images and the date of the survey can be found (but with difficulty - search the web for advice).
Which brings me to my fascination with the "green bits". I survey as I go and collect my data to get an overall picture. See here for more on amateur and crowd-sourcing surveying.
One frustration is that having got some good idea of the disposition of the channels and sandbanks, they may change in a matter of weeks, especially if there is a big storm. So this is really a job for life.
Another frustration is that sand and mud may contain harder bits: rocks, wrecks, pipelines, fish weirs, stakes, ruined perches,... Such items are often charted, but the chart information can be very out of date, also as sand banks change in height, new "hard bits" may appear.