The story is George MacDonald's "The Light Princess". Definitely fantasy. According to IMDB the BBC made a TV movie about it in 1978.
Here is part of that scene from the original story:
So the water grew and grew, and rose up and up on the prince. And the
princess sat and looked at him. She fed him now and then. The night
wore on. The waters rose and rose. The moon rose likewise higher and
higher, and shone full on the face of the dying prince. The water was
up to his neck.
"Will you kiss me, princess?" said he, feebly.
The nonchalance was all gone now.
"Yes, I will," answered the princess, and kissed him with a long,
sweet, cold kiss.
"Now," said he, with a sigh of content, "I die happy."
He did not speak again. The princess gave him some wine for the last
time: he was past eating. Then she sat down again, and looked at him.
The water rose and rose. It touched his chin. It touched his lower
lip. It touched between his lips. He shut them hard to keep it out.
The princess began to feel strange. It touched his upper lip. He
breathed through his nostrils. The princess looked wild. It covered
his nostrils. Her eyes looked scared, and shone strange in the
moonlight. His head fell back; the water closed over it, and the
bubbles of his last breath bubbled up through the water. The princess
gave a shriek, and sprang into the lake.