You'd have to change the ship into a submarine in order for that propositional change to make sense.
Could "The ship is sailing around on the lake" be an accepted answer? One of the hints for the "po" was "around" as well as "on." Okcydent also explains that "po" indicates motion.
Its's just po + Locative is much more commonly used with movement while na + locative with state/location. One can use na + Accusative to express some movement like changing between water bodies.
- Statek jest na jeziorze. The ship is on the lake.
- Statek płynie po jeziorze. The ship sails on the lake.
- Statek płynie prosto na górę lodową (accusative). The ship sails straight towards the iceberg.
- Statek wpływa na morze. (accusative). The ship enters the sea.
Unfortunately I used 'over' the lake . Even though I checked afterwards on the hint I guess 'po' hardly means 'over'.