If you do go out to see the Lava, which is really cool. If you can try to pick a moonless night, and check the Hilo weather for clear sky. I've never seen so many stars in my whole life. We stopped at the end of the road on the way back where there was a picnic table and just layed back on it and enjoyed the show. I would have loved to have a Telescope.
I would really suggest saving a couple of days to go over to the other side of the island, it really is a different world. Maybe stay at a B&B or or hotel one or two nights. Don't let the Big Island's size fool you, its a long drive from Kona to Hilo, even by mainland standards. But is absolutely gorgeous.
There are even two dive shops in Hilo, (at least last time I was there). There is a black sand beach at Richardson's Beach I believe just South of town that has really great shore diving. Its a bit of a surface swim out, but even that was pretty good because the snorkling there is absolutely fabulous.
Check out Dick the Diver for info on shore diving. You might also want to check out other posts in the forum for good suggestions on operators. Place of Refuge would be a good place to go to. Although for shore diving I think I would recommend some experience, or hire one of the dive guides.
Of the Kona boats I like Kona Honu Divers, because they are a small shop with a very big and fast boat. Jack's was able to fix a piece of gear for me, and I'm grateful for that. But I saw a bunch of people piling into a van for a ride to the boat. The boats also didn't look all that roomy to me.
Bob