B3 has an inboard raw water pump. There are 2 hoses 1 from the transom assy (raw water in) the other is the pumps output and goes to the t-stat housing. You need to pull both and I would plug the inlet hose with a emergency plug you can get from any marine store. Remove the belt then remove the 2 bolts holding the bracket to the engine, this will allow you to remove the pump assy from the engine and work on replacing the impeller topside. Be sure to reconnect the inlet and outlet hoses correctly (where you took the off). May want to mark one with a piece of tape.
You didn'y say what model boat you have but cruisers have 2 seacocks, 1 for the head the other for the AC.
Live long and prosper James T
1997 2400 SCR
5.7 Vortec / Bravo 2