The reason i highlighted this
possible magnet problem is that over here in the UK i do what we call magnet fishing,we use a smallish but ultra powerful magnet that can lift some amazing weights,these are attached onto strong rope and what we do is throw the magnets into rivers,ponds,lakes infact any water source that we think could possibly have somthing of interest,of course not all metals will work with a magnet.
But although me usually find just general rubbish that has been thrown in a river,but if you do it near strategic locations like a bridge or water crossing you could find anything from guns,swords that type of thing that could have been thrown away after a theft.I dont do this very often but mainly when we cannot get on the land with the crops still in,so on a nice sunny day its great fun to go out with your magnet and long rope and throw it around in the river or lakes.
Getting back onto the magnet issue,i have to keep my magnet in a special container other wise it could do damage to many things,they may be only about 2'' long and 1'' dia but these magnets are that strong they can pick up weights of upto 200lbs or more if you use a slightly larger one.
Another thing you have to be very careful with when using these high power magnets is they can trip your skin when they get stuck on a flat surface,but trust me they can inflict alot of damage .
Slightly off topic but its something i like doing when we cannot get out normal detecting,so we go magnet fishing instead
I really doubt that they would affect it too much, especially with an older machine. A newer detector with lots of computerized gizmos... who knows? Good question though.