Уou can add - Hydroacoustic buoy
Which could be placed on points in the sea .. by means of a ship or an airplane of reconnaissance of naval aviation
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Add a button to the ships in the interface - Hydroacoustic buoy
When the ship comes to the point this button becomes active and there is an opportunity to install a buoy. whose lifetime will be limited to 3-5 days ..
After 3-5 days, the maneuver will have to be repeated ..

he must scatter fog and give a review, with a radius of 25-150 km, track the movement of units through the point.
It must have invisibility properties, and it can not be destroyed ..

I believe you ..)
but there is a problem, as soon as planes or ships leave the point, the buoy apparently goes after them.
agree it's not very realistic ..

You must understand that a survey in the sea is extremely important in the conditions of war,
Especially when you do not have the resources to attack the enemy,
but enough to react to the landing of troops from the sea ..

To the word
in one of your strategies .. which perhaps you can call grand strategy game, a similar problem is solved by the fact that in the sea, at the point, 1 infantry unit is located ..
But in that game the infantry is generated, you do not have to spend resources on its creation, and as a result, it's not a pity ..

You propose to use planes but they are extremely important .. and losing them in these conditions is very easy ..

You do not need to draw this buoy, the main thing that the review was with a timer, which will conduct a countdown, suppose 72 hours .. at the end of this time the review at the point disappears ..

It will only enrich your game.