Maggies spot on.
It is the physics of friction if you are talking wind AGAINST tide as oppposed to wind OVER tide which was the original heading. That second circumstance will see flat seas. It is first scenario that sees the steep short seas.
So students - a definition of friction is 'A force that resist the relative motion or tendancy to such motion by two bodies or substances in contact'.
In this circumstance we have a body or water going one way - and a body of wind going another.
The mass of water at surface level slows down as a result of friction with the wind at sea level, and the greater the water flow or the greater the wind, so the greater the friction and the greater the effect.
As the surface water slows, so the mass of water below keeps going at speed. (As it happens, the water right down on the bottom also slows due to friction with the sea bed
, but not the water at mid depth).
This under surface water overtakes the surface water and the surface level friction of the wind (aided by the lesser density at the surface) causes mid depth water mass to curl upward - where it gets further exposed to the actual wind forces, thereby creating waves.
It is just the same - but in reverse - as water reaching a shore and friction causing the sea to roll over as they come in.
So the surface seas peak up, the higher they peak the greater the resistance presented by the wind, and despite what someone else refuted above, they can end up breaking back against the flow of the water.
But it is friction that results in those short steep seas.
You'll see it anywhere you get a narrow channel or tidal flow against the wind. Happens twice a day off our home port in the Solent and boats still get lost and people still die, especially when the winds are strong and the tide is strong in the opposing direction. Locals know you leave it six hours for the tide to reverse and it flattens out completely - so standing off until the tide is right is common practice.
You'll of course see further complexities ( re fluid dynamics) when you get cold water meeting warmer water (like the Gulf Stream). Due to density one tries to dive below the other...........but that IS another story.
So wind OVER tide when both going same way gives flat water.
Wind AGAINST tide gives the steep seas - due to the laws of friction.
Good luck in getting it over to the students.