The Manywaters
An enormous interconnected system of lakes, slow rivers, reed-shallows, and one coast, all of it warm and clear and unreasonably full of life. To a Faerie of any other race it looks like paradise, which the Undine find funny, since most of the Manywaters is a place you can only visit if you can breathe it.
Geography
Shallow, warm, and continuous — the lakes connect, and an Undine can cross the province without surfacing. The deep sinkholes at the province's heart are a different environment entirely, and the Undine treat them as a separate country they happen to own.
Tidewell
A stepped city built down the walls of a flooded sinkhole, with the noble quarters deepest and the visitors' quarters at the top where the air is. Undine society is fiercely communal and famously stubborn; the province's tithes to the Eternal City are always paid, always late, and always accompanied by an itemized complaint.
Relations
The province's unresolved problem lies at the bottom, where the sunlight quits and the black meres begin. The grindylocs were there first, and their territory runs beneath the Kingdom's border. The Undine, who consider themselves the water's rightful people, have spent centuries insisting that the deep does not count as part of the lake.