Depends.  The magnetic declination varies widely around the world, if
you do long swims, it might well put you a good deal off-course.  I
would guess you are not diving anywhere close enough to the magnetic
poles for compas inclication to be a problem.  But perhaps I still
missed something?

  /m

In article <47fee379$0$1233$5a62ac22@per-qv1-newsreader-
01.iinet.net.au>, bjcanham-nospam-@hotmail.com says...

Compasses are made for 5 different magnetic zones. There is a downward
force from the magnetic poles, and a compass that is used between a pole
and the equator will have some dip on one end. The other end of the
needle is counterweighted to compensate for this. A compass made for
zone 1 (the US) will tilt about 8 degrees down on the southern end in
Australia (zone 5). The southern end may drag on the bottom or the
northern end may drag on the top.

It's best to get a compass for the zone you're in. I would buy the right
compass instead of trying to save a few dollars by ordering one from the
US or Europe, unless you can order a zone 5. I've used a Zone 1 compass
in Zone 2 with no problem, but I have no experience with my compass in
zone 5.

In diving, we're not usually trying to correct to true north; we just
want some sort of repeatable reference.

There are global compasses available, but I haven't seen them in diving.