Any genuine expression of gratitude to your instructor for a job well done and thanking them for their time and effort in getting you started in the wonderful world of scuba diving will make their day. A heartfelt handshake with a smile and a "THANKS DUDE - YOU ROCK!" will more than adequately convey the message. If while you're right hand is shaking their's your other hand also offers a bottle of wine, a giftcard to TGI Fridays, or a couple of bucks, I'm sure that would be appreciated as well - but no where near as much as the sentiment itself!
That said, if you're inclined to offer a tangible recognition I'd go with something other than cash. Cash sort of says "at the last minute I thought I better pony up a tip" whereas a bottle/giftcard/etc says "I've really thought about - and appreciate - the value of the service you've delivered. So much so that I felt compelled to go out of my way to recognize it!" For me knowing someone made a special stop to pick up a decent $10 bottle of Australian wine would mean far more to me than if they shoved a hand into their shorts and pulled out a folded-up $20. Plus, I would get the benefit of that same feeling again when I drank the wine, as it would remind me of the gratitude of the student - providing further positive reinforcement. That's something you don't get when the person you "tip" spends the $20 on milk, bread, and cold cuts at the grocery store.
