Densuke Watermelons This page first published: May 2, 2005 · Updated: Jun 16, 2018 · by CooksInfo. Copyright © 2020 · This post may contain affiliate links · This blog generates income via ads · Densuke is a Japanese variety of watermelon. The melons are large, and tall, almost squarish. They have thick, hard skin, that is so dark green it’s often described as black. Inside, the flesh is red and very sweet.