Hi Pawel,
There isn't much info concerning the underwater caves in Ukraine. They are not well investigated, usually mentioned in the descriptions of some major nature phenomena.
So, Somyn Lake is in Turiysk Region, Volhyn Oblast, close to the Polish border. It's 56m deep, 6ha. There're karst underwater caves there. Some locals show the teeth marks on their legs left by a beast living in those bottomless caves
Most karst lakes of Volhyn region are connected with underwater channels. Some are 30m wide
There are some underwater caves near Aya Cape and Fiolent Cape, Crimea. This article is about underwater treasures of Crimea (in Ukrainian only)
Скарби підводного Криму
regards,
Andriy