Ahsoka tells Prince Lee-Char that the Gungans were the last of the Republic reinforcements and that no more help is coming. The decision to send the Gungans was made after the Jedi on Mon Calamari lost contact with the council on Coruscant. After that happened, the council determined that more clones could be outfitted for underwater combat, but it would take days to do so. The Gungans were only sent in because they had a standing army ready for underwater operations. More reinforcements could be sent, it would simply be a matter of time. In any event, Ahsoka would have no basis upon which to make an assumption either way.
Ahsoka's mask is not connected in any way to her oxygen tanks.