Garmin wind inst needs speed through the water input to give true wind direction. Why?

[Wing Mark]

But only in one direction, which is why it was a great example because it actually does apply when sailing too but I took that out of the equation to make the point that it actually is useful to know what the wind is actually doing.

Nonsense.
If the ground wind is zero and the tide is 4 knots Eastwards, you have 4 knots of True Wind enabling you to make progress North or South.
For sure your COG and SOG will have a big component of the tide taking you to Dover, but that will change in <6 hours.

 

[lustyd]

‘wind’ experienced is actually a northerly wind, but your boat’s heading has no effect on whether or not you are going to be able to sail

Of course it does, if there isn't any real wind then there isn't any wind regardless how archaic your electronics setup is and how much it tells you there is.

 

[Wing Mark]

I take it that we are still on your own electronically-equipped raft drifting north in the Alderney race at 4kn. The information you gave in #13 is confusing because the ‘wind’ experienced is actually a northerly wind, but your boat’s heading has no effect on whether or not you are going to be able to sail. If when drifting the boat feels a 4kn wind pressure, it will be able to sail in any direction, just as it would with a 4kn wind in still water with no tide. If perfectly accurate, the instruments would continue to record a TWS of 4kn from the north, whatever the boat speed. With this information, the sailor will know how and in which directions he or she can sail.

Armed with only ground wind speed (zero), it will be impossible to know that sailing is possible, if slowly, without information about the tide at that point, which, being only from a prediction usually, will be only approximate.

To be fair, unless the windspeed is several times the current, you really need to know both wind and tide.
So in reality, you know all the vectors.
Wind, tide and boat.
How you choose to process them is probably more complex than only caring about True wind or ground wind

 

[lustyd]

Nonsense.
If the ground wind is zero and the tide is 4 knots Eastwards, you have 4 knots of True Wind enabling you to make progress North or South.
For sure your COG and SOG will have a big component of the tide taking you to Dover, but that will change in <6 hours.

Indeed, and I'd know that with real wind, and it would be obvious I couldn't go West because I'd see there isn't any wind. My reading also won't change every few hours, or every time I hit a faster bit of water, so I can plan accordingly in my navigation