Hi Chris
I live in Guernsey, have done all my life and i like it. Have been diving locally for about 4-5 years and the local scene is quite active if you know the right people
Should be a link in my signature for the local BSAC club and we go out at least 3 or four times a week on our RIB (shortly to be upgraded to carry 8 at a push)
There are loads of locals who have their own boats so plenty of oportunity to do a large range of dives.
Not many 'breather divers that i know of but there is a group of techie divers who do the odd trimix dive. O2 and nitrox not a problem to get easily up to 300bar, Helium more of an issue. There are a few people that will pump it from their own supply but as it is a between friends sort of thing, they will only do it for people they know and are happy with. Not too hard to get 'in the know' though.
Diving itself covers whatever you want. Loads of fishy things to see, over 300 wrecks (i think) in local waters from 8ish meters to 70-80+. Reefs and walls are quite common sites to visit as are large callop drifts. a few dozen being the average catch per person.
Vis usually around 10M and mostly boat dives. There are some good shore dives but it is more faff than it's worth when the boat is only round the corne. Not that much in the sub 20M range to be honest though
tidal range is up to about 10M but once you get used to it, there isn't really a problem.
The 5year residency thing is a bit cmoplex but basically, you get a 5 year licence to work. This may be extended to 7 years on application depending on the job you are going for and how 'valuable' it is to the island. Once the permit has run out, unless you are loaded and can afford to live on the non-local housing market (which isn't cheap), or considered local (which still isn't cheep) by that time you basically have to leave. Like i say, there are lots of ifs and buts but it is basically to try to keep jobs available to locals where possible.
Before i get flamed on the above thing, yes, it is harsh!
If you want any more info on anything, let me know, happy to help out
Edit - when are you over? Can meet for a chat if you like. Local club also meets wednesdays so you could get a more varied opinion on the diving.