Vessels at anchor have right of way. It's really as simple as that (unless it's a drift, which is another story). Still, I can't leave it there, can I?
We do our checkouts out of Panama City Beach, Florida, where there are more than a few sites. That said, it is often the case that only a subset of them will be very dive-friendly. The silty water coming out of the bay can drop the viz to levels that make checkout dives problematic. Additionally, some of the sites are more useful for students' first saltwater dives. (The truss bridge spans they've sunk out there are great, as we can keep the basic students "riding the rails" on the top, where it's very difficult for them to get lost or deep.)
Anyway, on one trip out there, we ran into a belligerent fisherman. He was fishing on a bridge span, anchored to the in-shore end. We went to tie in to the off-shore end (as it's certainly big enough for the both of us). He started his engine and raced to cut us off. We went to another side, and he cut us off there, too. (His language was not such that I will repeat it here.) After some... discussion... he said he was going to call the marine patrol, to which our captain answered that he already had (or something along those lines). All in all a wonderful situation thanks to a most likely drunk fisherman with strange ideas about us "scaring off all the fish". (For the record, the fish don't seem to care about divers, unless for some reason you're trying to catch a nurse shark or a goliath grouper.)
On another occasion (at the same span, actually), a couple guys were out fishing. We approached and explained the situation (that we really couldn't bring the students anywhere else given the conditions and profiles). We even offered coordinates for some other sites which usually have much better fishing than the spans. The guy graciously (if understandably a bit less than completely enthusiastically) said that he'd pull his anchor and let us have the span. When he tried to pull it, however, he found that the anchor was fouled. We sent our boat's divemaster down, freeing his anchor, and sending them along their way.
You know, he was quite a bit happier as he left. I suppose he realized that had he not been gracious and yielded the site, he would've lost an anchor to the wreck. (Okay, I was just *slightly* bummed that he hadn't, as I'm usually the one who comes up with lost anchors.

) He was nice about the site, and so, he had friends around to go down and give him a hand. Just goes to show, you should always be nice to divers.
(Of course, there have been many occasions where we've gone out to dive a site and found people out there fishing. The normal procedure, when conditions allow, is to pull up the waypoint screen and pick another site.)
What really bugs the tar out of me, on the other hand, are those few occasions we've had when our boat is tied in to a wreck and along comes another dive boat. We don't do our checkouts off six-packs. Sometimes, we may have two dozen divers (or more). It's hard enough to manage three class groups of six students each (with instructor and divemaster per group), timing the entries to try to keep only one group on the line at a time. Throw in another boat sending divers down onto a small site and it becomes... well, let's just say that it's not the most relaxing thing in the world to keep my group together and distinct. :biggrin: