Which sensor to measure moisture in beach sand?

I'd like to deploy a sensor in a beach and measure when the sand is inundated by the tides. Which sensor would you use for that? The logger should be deployed for several months.

Thanks a lot for your help!

Wouldn't local tide tables give you the same info? The rotation of the moon around the earth causes the tides, and that certainly isn't changing.

Also, I would think securing the sensor for several months against very strong changes that occur daily (twice a day?) would be difficult.

Depending on where the sensor is located, the amount of tidal change varies quite a bit also with the shape of the coast line, and also with any storms that might be occurring. We often hear of tidal surges when hurricanes, tropical storms, tropical depressions, etc. are occurring in conjunction with the tide. Beach erosion can be drastic after an event like that.

I think the fact that the tides are entirely predictable is lost on a great many people, at least here in the UK.

The news often has stories about people being rescued from local beaches etc after having been 'cut off by the tide'.

One local offshore island, connected by a tidal causeway, now has a very large sign and flashing lights warning people that the tide is comes in.

We've been watching some of the TV shows where people film themselves cruising up & down the England canal systems, sometimes going up & down the rivers. One of the rivers had very strong tidal flow, the canal boats with their fairly small diesel motors really had a hard time going against the flow.

Ok, maybe I was not clear enough... of course I know what tides are and that they can be predicted (I'm a marine biologist). However, in an experiment, I want to monitor condtions at samples deployed in natrual beach sand (10 cm into the sediment). The experiment is in the Mediterranean, so changes in wind direction and force and even air pressure can have a huge effect which is not (easily) predictable.
So to specify the question: I'd like to know, when the space between the sandgrains is filled up with seawater and, if possible, also the water level above the samples (can be calcuated when pososity of sand is known).
I thought that the first requirement can be met with some kind of moisture sensor, the second one is more difficult. I found the MS5837 available in sufficient accuracy, but this sensor is relatively expensive, difficult to waterproof (I only found waterproofed sensors at bluerobotics) and seems to drift.
Any recommendation?

Seeing as how you are dealing with salt water. One could possibly measure the amount of current flow between 2 points through the sand/salt/water at the desired depth and see if there is a difference in current flow at the various tide levels?

Salt water is EXTREMELY corrosive to electronics and metal objects, especially when current can flow. Keep in mind that the contact between two dissimilar metals forms a battery when exposed to salt water. There is almost zero chance that an amateur-built circuit will survive for months, even if only exposed to ocean air.

That said, I suggest to try noncorrosive probes, like gold or platinum, and use infrequent bursts of AC signals to measure conductivity.

However, the most reliable way to measure whether water is above a certain level is pressure. A bulb of rubber or plastic could be placed at some level, with a plastic tube leading to a sensitive pressure sensor.

Yes, pressure would probably be the best way for it (althought monitoring salnity would be also very nice). I could not find any description of a waterproof water sensor in a flexible waterproof container (even a small PET bottle coudl be suitable)…

The tides are predicatable, but its not just tides that affect seawater level - storm-surges of
upto a metre or more happen, especially in shallow seas. Wave action has a massive effect too!