Do yourself a favor and don't do it.
Think about what you just said - gotta get low and shoot up. Well guess which way the lava is flowing - down.
Plus, it moves a lot faster underwater on the slope than above water up on the plateau. Not to mention the fact lava reacts violently with sea water, and if you are close enough to see the lava you're close enough to have unstable land further up the slope/topside come crashing down on you.
I'd think it's particularly suicidal if you're thinking about going it alone.
It's one thing to be back off the flows 100 yards out on the surface, it's another to be within 10-15 feet of that stuff underwater when you've never been around it in that environment before.
I'd say you'll have better luck (not to mention a better chance of getting home alive at the end of the day) if you buddy up with one of the park rangers, find one who dives, and get yourself in good grace for the next time they do a little diving out there. There's lots of folks in the park ranger service that are certified divers, and I think they have the SCRU group out at least once or twice a year when things are active to check things out.