Let's look at some of this.
huge thanks for these detailed replies. Just to give some background info, and maybe correct how I initially presented the scenario:
its actually 2 brothers. they are driving on a road next to a river. huge noreaster storm, its been raining for days, the river is rising.
It's not so much about how many days it's been raining, but how much rain per hour/day.
they drive over a lowlying part of the road and suddenly the the road floods.
Any predictable likelihood of the low portions of a road flooding, and the road will be closed. You need to understand the difference between a flash flood and regular flooding. A flash flood is a massive wall of water, raising water levels in a matter of minutes. Other then in the southwest, flash floods are caused by the sudden release of water due to something like a dam failure. In a flash flood situation, you're just going to go. You need to see a video of the initial flood, not the after effect to understand the force. In this situation, the car will go right with the water.
But in the situation you're describing, the water level on the road will rise steadily, but somewhat slowly. There's not much lateral force against the car. It typically arises from driving onto a section of road that is flooded, and underestimating the depth of the flooding. You get into a foot or so of water, and then your engine stalls.
The brothers climb on the roof of the car,
Which implies they didn't bring anything with them.
waiting for the fire dept rescue boat, still about 50 feet away.
From 50 feet, I can throw a line. Put the rescue boat a lot further away or get rid of it. Fifty feet below the car and they won't sink before they get to the boat. Fifty feet above the car and you do a quick run around the car and a hook. Still pretty likely to find both brothers.
the car they are on begins to rock and then float in the flooded road.
This is going to give the brother some time to work in. Have him watching for logs that might work. Having the other brother being willing to go, then not, would be good technique.
the brothers are tossed off the roof of the car.
No, they lose their balance and fall. Likely to be some scraping against the car. Upstream fall is more dangerous then a downstream fall as the pressure of the water keeps pushing you against the car.
the older brother is only 16 but on the swim team,
This means the brother has some significant water experience.
younger brother is autistic and can't swim.
Okay, here's how I'd do it in this situation. First the shoes go, although save the laces if the shoes have them. Strip down to the shorts, and rip the shirts in half. Tie them together, with the pants. You've now got a short length of rope. Nothing great, but better then nothing. Tie one end around waist of younger brother, with plan being for the older brother to pull the younger.
Older brother tries to rescue younger brother but ends up having to let go
Have older brother with a definite destination in mind, such as a tree floating by. Have him have to push his brother into the water, and then jumping in after him, holding onto the other length of the cloth rope. He can drop his hold on his brother through cold, being banged, or almost drowning himself.
and younger brother drowns. older brother manages to grab hold of a tree and hang on until he is rescued.....
i like the idea of watching some of these type rescues to get ideas...
Best of luck,
Jim Clark-Dawe