The ‘cement’ is probably a zinc sacrificial anode on the rudder, to save the rest of the running gear. Looks like that poor boat slept in the salt water. Glad it’s getting a new life.
Currently Boatless, searching for next boat SN 196...?
99 MasterCraft SportStar CB
98 Moomba Boomerang 310 HP 350 Carb
88 MC PowerSlot 240 HP 351 Carb Black on White