The Frenessican Sea is a place mentioned in Nennius. It is located in the ‘land of the Picts’ and is spoken of in conjunction with the Orkneys.
It seems obvious to me the name is based on some misread manuscript reading Fl Nessa (the River (fluvius) Ness). This is a common enough when it comes to documents such as the Peutinger Table (a late-Roman map of Europe), laden with obscure and incomprehensible names.
The Frenessican Sea is on this basis Loch Ness.
Interestingly, the early medieval poet Béroul, in his entertaining Roman de Tristran, mentions a ‘mer de Frise’ (implying a ‘Frisian Sea’). This name may go back to a ‘mare Fressicum’, which in turn is nothing more than a misreading of Frenessican.