I went through this myself years ago, and ultimately decided on the Pelican 1510 vs a backpack (Lowe-Pro and others have many options. I was quite taken with fstopgear.com although they're pricey).
Several years later, I am happy with my decision... Here are my reasons why:
I use an Aquatica/D7000 system and dual Ike strobes etc. Add it all up and it's surprisingly heavy. (Upwards of 35 pounds). This would get damn heavy totting it around all day.
Due to the weight, it's entirely possible that some airline will insist I check it. With the Pelican, I know my stuff will be safe.
The Pelican meets carry-on requirements and I have never been challenged.
My only other comment would be that if you do opt for a Pelican, I seriously urge you to consider a roller case. With the case, my kit weighs 42 pounds.
The other thing to consider with a case vs backpack is that a small pack will qualify as a "personal" item. In addition to my 1510, I carry a small Lowepro pack with a sleeve that protects my 15" Macbook Pro, and has enough space and pockets for another camera body, dive computers, travel documents etc. It's quite small, but easily fits in under the plane seat.
I dive a couple of times a year in conditions like you describe... black, tannin-laced water. Last year I decided I was going to get some decent images of this little wreck. My previous experience was you have described yours.
So I added a little iTorch V10 and mount it on the cold shoe. I found that on the lowest white setting I was still getting backscatter. On the red setting my d7000 could focus fine and the scatter much less
The light was cheap but my one complaint is that it eats batteries so get a spare!