The simple answer is that it is. The reason for this is that the Rema equates Shabbos and Yom Tov in regards to bathing in warm water (OC 511:2). Just as it is forbidden to bathe in warm water on Shabbos, even in water that was heated before Shabbos (OC 326:1), so to it is forbidden on Yom Tov.

Even bathing in cold water on Yom Tov is problematic. There is an accepted Minhag not to bathe in cold water on Shabbas (Mishnah Berurah, OC 326:7, #21). The reasons for this Minhag apply equally to Yom Tov and therefore the Minhag applies there as well. There are Poskim that hold that this Minhag only applies to bathing but not to showering, and therefore permit showering in cold water if the person is uncomfortable from the heat (Shemiras Shabbos ke'Hilchasa, 14:11).

This is a complicated area of Halachah and there may be extenuating circumstances in your case. Please consult your Rav for an actual Psak.

Kol Tuv,

Yonason Sigler