Cost of Breathe Water is 4 for the first minute and 2 for each minute afterwards. Which means that it costs Hawk 3 and 1 respectively.
Assuming he's given a full hour to rest between attempts and people are able to get out within one minute, Hawk can cast on three people at a time. The latter looks unlikely, so, at full strength, he can give two people each three minutes to escape, look around, fight, whatever.
Reminder: he recovers 1 FP/5 min and is currently at 0 FP. So he'll need 15 minutes before he can even cast this on one person for one minute.
I recommend someone take Eabd and go scouting now, before the place is underwater, as it probably will be by the time Hawk and Lord Arron are able to do anything useful.
Two people in armour (plus golem), fighting in formation, shouldn't have much of a problem fighting the Things. We just had problems because we weren't in formation, and the non-fighters got pinned down away from the others.