As someone who is covered more than 30% in tattoos, I say please don't just go get a dive flag tattoo. Most of them look bad and I suspect a lot of people regret them a few years down the line.
IF you live the "scuba life" or it's a major factor in your life, a dive flag tattoo might be a good choice but generally speaking I think most of us fall out of that group of people.
If you DO decide that a permanent option is the right choice for you, take your time finding a good artist that has clean lines (meaning not "wiggly") and good color applications that you like. Design something unique and please don't just do a flag (no offense to those with just a flag) do something more. Make it more specific to your own diving choices and experiences and it will be something you like for a long long time.
As for temporary tattoo options, sharpies are great. We used to do that with my daughter when she was younger every summer. She'd come up with a design and I'd sharpie it onto her and it would last anywhere from a week to 3 weeks, depending upon how much time she spent in the water. I also know a lot of tattoo artists who do this for people, though they do generally charge some nominal fee for it.