Bunker gear w/o life preserver has a ridiculously short time to stay afloat. The SCBA might provide some air for a while, but they're SCBA's, not SCUBA's. And that's if the facepiece is on when you fall in.
When we went to a boat/pier, depending on our due, I'd have maybe 2 guys geared up in case someone happened to be on the boat. Everyone else absolutely not, and make sure we had ropes, pfd's, etc. If we were FAST, guy would be in drysuit if it was cold out. And if we were FAST, I'd make guys that weren't actively operating to get the fuck off the 3' dock that shifts when there's fat firemen crowding around.
Look at how the guys on the boats are dressed. This is what they do full time. I'm not losing someone at a car fire that happens to be on the water.