Yes.
Always tip your scuba instructor.
The best tip your instructor could possibly get would be a bottle of golden single malt Scotch whiskey. ALL dedicated scuba instructors enjoy sipping single malt Scotch upon the completion of their instructorly duties.
If your instructor is any kind of truly superior instructor at all, s/he will prefer Oban.
A bottle of Oban says "Thank You Dive Instructor!" to all those of us who care for our students. Contrary scuba instructors may enjoy Irish Whiskey (..."it's MY Island!" "You mean Ireland?" "Yes. It's Mine!"), however, only strokes drink blends.
If your instructor has the unmitigated temerity to refuse your tip, please PM me.
I will immediately provide an address to whence you can ship said tip to a far more deserving scuba instructor!