Me and Rachel Pollack, seems like I'm constantly referring to her, but in Seventy Eight Degrees of Wisdom, she has both pictures for Death and Knight of Cups in the discussion and description of the Knight. For all that, she doesn't have much to say about that except "he resembles Death, symbol of Transformation," and this about the conflicts of the Knight.
Basically this Knight is full of conflicts, between his committment to action and involvement of his questing nature and the passivity of water, his inexperience and undeveloped state. He has a hard time reconciling this contrast, so if the passive side denies his committments, he never allows his imagination to produce anything. His helmet and feet are winged, but his horse is slow.
The Death connection is following a genuine vision rather than escapism to transform himself by going deep with in to resolve his inner conflicts. It could be "the precise moment at which we give up the old masks and allow the transformation to take place" I couldn't find any reference in the Death description, but was struck by the similarity of terms, "lethargy" in Death and "passive" for Cups, refering to a desparate battle of the ego to avoid change (or in the case of the Knight) committments. The river might be "the unity of change and eternity" So this card could mean it's time to take on responsibility and grow into our adulthood, become more mature and make something of ourselves.