"Caligula was a man of many passions, and he indulged nearly all of them, including his passions for chariot racing, theatrical performances, gladiatorial games, and ships. During his brief rule from A.D. 37 to 41, he had two enormous ships--a sailing ship and an oared galley--built and anchored on Lake Nemi as pleasure craft. Pillaged and deliberately sunk later in the first century, they were recovered in a feat of engineering sponsored by Benito Mussolini in the 1930s, but destroyed during a German retreat in 1944." http://www.archaeology.org/0205/abstracts/caligula.html
Here an interesting description, written by Colonel Maceroni, in 1838.
A very singular piece of antiquity exists in the lake of Nemi, of which I have never seen any mention made by travellers. On the north-east side of the lake, in about forty feet water, lie the ruins of a large floating palace constructed by the Emperor Claudius or by Nero (in fact, Caligula). The lakes of Albano and Nemi are the craters of extinct volcanos, from which, at the time when they were covered by the sea, has proceeded all that vast quantity of tuffo and puzzolana which covers the Campagna di Roma. The high, steep banks of these circular lakes, covered with most beautiful trees and villas, cause one side of the water below to be constantly sheltered from the wind. Hence a floating habitation will give the choice of shade and shelter, or sunshine as the season may require. The way in which I became acquainted with this sunken palace was quite accidental. Being one evening in my punt about to lay some eel lines, the fisherman whom I employed told me, that the best place in that vicinity was "about the old palace". I stared and looked about. "What palace?" said I. I see plenty of houses and cottages, and ruins, on the hills around, but they are not even quite at the water's edge. My man rejoined : "I mean about the wooden palace under water in which the Emperor Claudius used to live." Delighted and excited by this announcement more than I should have been by the capture of a thousand eels as big as the mast of a ship, I hastened to the spot, but the declining sun had sunk below the high crater wood-clothed margin of the lake, and looking down into the limpid waters, all seemed dark and blue, and nothing could be seen but the hills and trees, and my own anxious physiognomy reflected in the watery mirror. However, I laid my lines, the hooks being baited with the thighs of frogs, and next morning I found fourteen eels, all about a pound a piece, and some of three pounds weight. Moreover, a brilliant sunshine enabled me to see the sunken palace, which appeared to be about one hundred feet square and fifteen to twenty feet high. How did I then regret not having the command of a diving-bell! What most curious and precious objects of antiquity might not be found in the interior of this construction? But this discovery I must leave to some future traveller, who may have the means of causing a diving-bell to be constructed at Rome, and know how to use it. As for myself, I mentioned the discovery to General Miolis, the imperial locum tenens, or Governor of Rome, and also to the learned antiquary, Mr. Norviuse de Monbreton, but nothing was done.
From the MEMOIRS OF THE LIFE OF COLONEL MACERONI. LATE AlDE-DE'-CAMP to JOACHIM MURAT, King of NAPLES etc.
LONDON, JOHN MACRONE, ST. JAMES' SQUARE, MDCCCXXXVIII