You cannot go inside — it's a photo stop only. Vendors along the lakeside road push camel/horse cart rides and 'boat tour' offers; there's no legitimate boat tour to the palace itself, it's a scam pitch.
Sitting in the middle of Man Sagar Lake, which was formed around 1610 when Raja Man Singh I dammed the Dravyavati river, the palace itself was largely rebuilt and enlarged in its present form in the 18th century under Sawai Jai Singh II, with later use as a hunting lodge for royal duck-shooting expeditions under Maharaja Madho Singh I. Only the top of its five stories stays above the waterline, the lower four remaining submerged, and its symmetrical red sandstone facade is a well-known example of Rajput palace architecture. A major restoration in the 2000s reclaimed the surrounding lake ecosystem, but the palace interior remains closed to visitors, who view it only from the shore.