I've talked to a number of lawyers informally. I get the spectrum from 'you need to get a signed waiver every year' to 'that should be good enough'.
I deal with lawyers every day. For the past 25 years. My lawyers, my company's lawyers, my client's lawyers, the government's lawyers, etc, etc. I'd say easily more than 1,000 different lawyers over an insanely wide range of situations. The ONE thing all of my experiences with lawyers have in common is that I've never, ever, ever heard a lawyer say anything even remotely close to "that should be good enough." Ever. Even when asked to critique their own work-product they will still tell you all the additional things that could be done.
Were I your lawyer, I would advise you to get a new waiver per event. If I were a plaintiff's attorney I would start drooling if I heard you had a "once a year waiver." If I were your insurance company I would drop you like a hot potato.
Every dive/day/site/conditions vary. How can someone be expected to waive - or ask to have waived - in January, liability for a dive to an unknown site/depth/conditions/etc that might happen 1-to-11 months later. All you need is some schlub to die on a 100ft dive in November, and have his attorney point out that:
1.) The waiver was signed in January
2.) In January the schlub had never dived deeper than 60ft
3.) Even though the diver's experience may have grown significantly over the 11months, no one specifically pointed out to the schlub on the day of the deadly dive that the deadly dive varied greatly from the schlub's experience 11months earlier
4.) Accordingly, the schlub could not have possibly released liability, having lacked the requisite information upon which to make an informed decision to do so in 11 months earlier.
There's a dozen or more other things a plaintiff's attorney could add to his case. Did the club talk up this dive as being a big deal all year? Did anyone ever make fun of someone thumbing a dive? Do your members regularly compare numbers of logged dives? Do your members brag about who has done more dives? Deeper dives? All of that fosters an environment of peer pressure and duress. With a once-a-year waiver all of that stuff becomes cumulative, with the diver never really having a chance to decide to "opt out" of doing the dive. But, if you put a waiver in front of them before each event... you get the diver to reaffirm their release of liability with full understanding of the dive, the conditions, their skills, etc.
A lot easier to defend.
We get signed waivers from customers that come out two days in a row to the same wreck. We don't often run two trips in the same day, but if/when we do we'll have anyone who's joining us again in the afternoon sign another waiver... even if they never set foot off the boat when we went back to the dock for the second trip.
PS - I wouldn't take directions to a gas station from a lawyer who ever uttered the phrase "that should be good enough" relative to ANYTHING.