[Geezer (OP)]

While lying on a beach, I can remember watching small clouds heading south over the land, but as soon as they were vertically above the sea, they disappeared. What would cause that?

[CliffordK]

Odd,

Cloud formation would be dependent on temperature and humidity at the elevation where the cloud forms.

The higher the temperature, the higher the dew point (or condensation point).  I suppose that is why people talk about fog burning off mid-day.

This would drive clouds back from the tiny suspended droplets back to vapor which would be invisible.

I would have expected the humidity to be higher over a water body, but perhaps it depends on the direction of the wind.  If the wind is blowing from land to sea, then it would be driven by the humidity over the land.  If it was the other way, then it would be driven by the humidity over the sea.

[Geezer (OP)]

You know what! I think I have it the wrong way round  [:I]   (It was a very long time ago.)
 
I think the clouds were coming in off the sea, so it was probably just caused by heat rising off the land.