Hi Sean,
this won't exactly be the help you're looking for, but have you checked out your iMac's IRDA?
Back since the days of PB2400's, I've had many problems with the IR port on Macs. And since I gave up connecting my PalmIII(!) via IR to my PB2400 back in 1999, I've pretty much decided IR ports on Macs are defunct out of the box. (With the Pismo (PBG3/2000), I didn't even bother experimenting after a couple of failures!)
As for dive computers, I've been connecting my Suunto computers via serial-USB converter and VirtualPC with Pismo and PBG4, and moved to Parallels on my Air recently.
If your IR port is working, ie verified thru other PDAs etc, then it should be only a software/setting problem. Then it might take time to figure out, but your problem should be solvable. I hope things work out.