Corrected entry: When Indiana flips the knight's coffin over into the petrol, and they're underneath it, it should be sinking. There are holes in the top that the rats come through, yet this stone coffin floats so well.
Corrected entry: When Indiana breaks in through the window, he lets go of his whip, yet he has it through the rest of the film despite us never seeing him retrieve it.
Correction: Just because we don't see it onscreen doesn't mean it didn''t happen. He could have retrieved it anytime after killing the guards.
Well... don't they immediately leave the room, before their escape is foiled?
The moment when Indy picks up his whip, it's actually shown. While Henry Jones is studying the broken jar, Indy is coiling the whip in the background.
Correction: It's not a stone coffin. It's wood. That is evident by the ease with which Indie takes off the lid then flips it in the water. A stone coffin would not have holes eaten through and termite burrows.
Isn't the point that you can't have an air pocket if there are huge gaps? A wooden boat still sinks if it's got a hole in it.
Spiny Norman
Almost all wood floats, and a wood boat, even with holes in it, will still float, unless there is other material (like metal or stone ballast) that makes its total density greater than water. A wood coffin might have a lot of metal attached that could cause it to sink, but we don't see that so it would float, even upside down and with holes in it.