Structural Assignment, plus, Rescue, Squad(depending on where it is), one or two boats depending on where it is, BN, RB, SB, Tac. If it is obvious this will be out of reach of land units, just the boat. One of the units assigned, Engine or Truck depending on where is assigned as water rescue unit.
If Rescue puts divers in the water, a second rescue is assigned to back them up.
Just recently amended, any reported floater(DOA in the water) receives a full water rescue assignment.
I'll use the 79St Boat Basin as an example. Engines 74, 40, 23, Ladders 25 (Water Rescue) 35, Battalion 11, R-1, Tac-1, SB, RB, Marine 1 and Marine 1A. If R-1 puts divers in, R-4 is coming in as a back up.