Depth (and potential DCS). You have no idea what depth the boat has sunk to. So let's say it's at 50m, it takes a long time to surface from 50m and if it's already taken you 1 minute to get out of the boat, you're fucked again. Not to mention any DCS you may suffer if you've been breathing compressed air down at 50m for very long!
If the boat is at 50 m the chances of getting rescued in time are probably so low that it may even make sense to try as a last ditch effort? If you remember to breath out on ascent. Hard to say...