Open is standard but I'd fold to the 3b. If BB is that huge of a whale then I guess call is ok, but then you really need to jam over the river bet yourself. As played I do not understand how SB got here with the nuts but he probably did... To me the biggest issue in this hand is not jamming the river yourself vs a whale with a good flush on a 4straight board. Get max value!

Why can't I post threads?

I would have bet about 30% of the pot on the flop. No way would I fold to his river raise. Sure he could have a bigger flush, but there are so many more hands you beat. He could have rivered the straight. He may just be betting an ace. He could even have a lower flush. You played the hand weak by checking down so it's also easy for him to just bluff that river. He probably would never think you even had a flush draw.