Many dives can be prevented by just not having the right tiny tool. Most experienced divers will carry a save-a-dive kit with all of the tools necessary to fix common problems.
A lesser known life hack is that rusty chrome-plated steel tools can usually be brought back to their original finish just by rubbing them with aluminum foil. Sadly this does not work on our chrome-plated brass scuba gear.
Wrenches
The most common problem is a leak, which can almost always be rectified by removing the problematic part. Frequently just removing and putting back the part can solve the issue. Sometimes the problem part can be hard to access, like in the case of an air pressure transmitter, so a thin wrench is ideal.
Adjustable Wrenches
Channellock – 6 Xtra Slim Jaw Adj Wrench
Hex Wrenches/ Allen Keys
Stainless Steel
Multi-Tools
I have yet to find a great scuba specific multi-tool but some are worth having.