Can internal waves be seen from the surface? If yes, what is the minimum elevation at which they can be seen? I have also heard of "dead water", when internal waves resist smooth sailing of ahips. If they are so superficial, can a diver appreciate them?
Internal waves can be seen only in special circumstances, when their vertical motions and associated convergence and divergence of water are near the surface. These motions compact or dilate waves at the surface, changing their appearance. So what you are actually seeing is the reflectivity changing in the surface waves. Internal wave effects, to the extent that they show up at all, are relatively large and so you would need a broad view (i.e., you would have to be high above them, say >> 500 m). They can be seen from satellites.
Internal waves exist beneath the surface of a body of water and oscillate much more slowly than surface waves. Both types of waves respond to the restoring force of gravity, where water is displaced in a fluid column that has a vertically varying density. For surface waves, the change in density is between the air and the water, which is a very large change. When the water is displaced at the surface (tossing a pebble into a pond) the pocket of air displaced downward "feels" the much denser water around it and the water surface rebounds upwards, creating an oscillation carried off by waves. For internal waves, the change in density is much more gradual (due to temperature changes, for instance) and the waves produced by water displacement have a much lower frequency.
Streaks commonly seen on the ocean surface are associated with wind effects.
"Dead water" occurs when the change in density of the water (stratification) is very sharp and very near the surface, in particular, near the keel of a boat. This happens when relatively fresh, warmer water overlies more salty, colder water (like in fjords). A boat travelling thru this water will generate internal waves, carrying energy away from the boat (like the typical surface waves we are familiar with). This energy is supplied by the movement of the boat which in turn is powered by its motor. Thus, instead of just generating energy for the boat velocity and surface waves, the motor also generates internal wave energy, which seems to "suck" away power. The internal waves cannot be easily seen and so early boaters were mystified.
You would have to be a very patient and observant diver to experience internal waves. The vertical movements of the waves would displace a diver a just little bit (~body length on a good day) under typical conditions, but it could take many minutes.
Hope this helps. Fell free to ask a follow-up question.