Sorry I've been so slow replying. It's been a long travel week.
This is all VERY good advice. Most of it I "kinda" already know, but it helps me to hear it reinforced by you experienced folks.
Someone asked about how I shoot. With the SP-350, it's been mostly manual mode with manual flash (which is the reason I got it after the Sea&Sea DX3000). I would never claim to be a good photographer, but I know which levers to move to influence the pic in various ways. My current goal is to get better at setting things up closer to optimal the first time.
On the view finder (versus live view), it sounds like there are opinions all over the place. Same with TTL. Both would be new to me. I've ONLY shot live view with manual flash. I'll probably just have to try both the view finder and TTL to decide for myself what my personal preferences are. Alcina, I think it will probably be like you suggest: both are just new skills that become non-factors once learned.
RTRSki, your comment about "you can afford diving or photography" made me laugh out loud. My purchasing decisions are often measured in Cozumel trips... such as, "yeah, I could buy that new car, but that would cost the equivalent of a Cozumel trip every month." I'm justifying a new camera purchase by telling myself that it's one of the things that makes the diving experience extra enjoyable, both during and after the dive. (And no, I don't have the new car.
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I'll share a little more on my motivation for the new purchase, and BurBunny touched on it with her "I was an Idiot"/DSLR post... A DSLR should offer better pics with fewer shots and time. My wife is very patient with me, but I absolutely love to watch the critters do their thing while I take pictures. With my slow strobes and P&S cameras, that means something like... take a shot, wait 10-15 seconds for camera write and strobe refresh, setup new shot, oops he turned his butt to me, reposition, take next shot... rinse, lather repeat. With fast moving or transluscent subjects it can take a lot of P&S shots before the moon, stars, and planets align to get a reasonable pic. If I can get a couple of good shots in a shorter amount of time, my darling Scuba buddy will generally be happier with me on fast current dives. That said, a faster/newer P&S with better strobes might accomplish close to the same result.
And so now I've been consolidating the advice about having both a DSLR and a P&S and thinking that I might upgrade strobe(s) and P&S camera in the short term, but make sure to buy strobes and arms that will complement a SeaTool housing in the long term.
Thanks again for all the great thoughts!