The conventional wisdom is to go for overkill. I have had 100m watches at 150 ft (essentialy half their "test depth") do fine but then I have had others leak at the same alledgedly conservative depth.
I would avoid 30 meter rated watches since you could conceivably push and exceed their limits even on rec dives and I would suggest they are best limited to showers and hot tubs. 50m rated watches are basically the same.
Then you have other issues that come into play. Some watches may be rated to 100m or 150m but only if you do not push any buttons, which will cause them to leak even at shallow depths (I had one of those). And then you have watches that are rated to 100m in cold water, but will leak at 106 degrees in 2 ft of water in your hot tub (I also had one of those).
Personally my dive watch is rated to 200m which is about twice as deep as a I ever plan to go, has no buttons and a locking crown which is about as reliable as they get. In general though, 200m rated watches do not have the annoying (and totally impractical) button pushing limitation and tend to make far better dive watches regardless of design.
But then again all bets are off if the case is ever opened unless you send it back to the factory to be resealed and pressure tested. So self winding or self charging watches are preferred as battery changes are otherwise fairy expensive and protracted affairs.
The other conventional dive watch wisdom is go cheap and buy a Casio G-Shock or Timex Iron man for under $50 and dive it until it dies, which as it happens tends to be a very long time. Several years ago I found a Timex Iron Man at 70 ft that had been there for at least 8 months. It was still running and it is in fact the watch I dove with when the expensive ones went back the manufacturer.
Currently, the king of cheap but practical dive watches has to be the Timex Helix. they are ugly as sin and need a longer strap for cold water diving, but they make a great backup depth guage and automatic dive timer in addition to telling time. And they go for $20 at Wal-mart if you can still find them or $30-$40 on e-bay if you can't.
