I borrowed a book from the library this week: "Submarines of the Tsarist Navy: A Pictorial History", Edited by Spassky and Semyonov, 1998
This is a wonderful little book, delivering exactly what it promises with pictures and diagrams of the Russian Imperial Navy's submarine fleet up until the Bolshevik takeover. Lots of great Aquanef inspiration inside and two designs which have caught my initial attention are:
Osetr (Lake) Class, 6 boats in class, built by a US firm 1903-106
Delfin - the first Russian battle submarine, launched in 1902