fbp says pretty much all here...Also as an FYI, the 7070 (which is what I WAS planning to upgrade to) is being phased out.. from what I've heard...
They're all moving to a cheaper DSLR in the $600-$900 pricing...
Nikon is coming out with some new models pretty soon.. Nov/Dec, so as pointed out, it depends upon your pricing and really what you want to do with it..
the 5050 on up are good enough to keep you going for a long while.. at that level...
Most of the lag time will be in the lower end, but believe it's pretty much eliminated at the 7070/8080 range and the start up time doesn't matter UW as you turn it on before you dive and turn it off after you're done.. sleep mode, if you are shooting all the time, might occur, but then you're not really shooting "Action" shots all the time.. soo...???
Don't forget strobes if you'll looking to get into it..
Like I said, you need to figure out what your parameters are, then get the info..
I SO hate to see folks spending $500 or so for (film or digi) housed fixed-focus limited auto-feature UW cams...unfortunately, in UW photography (as in Sail racing), MONEY goes to Windward...
IMHO, the trick is to get into an EXPANDABLE system... I started with an OLY C-5050 (wished I had waited for the 5060 with much better WA), and an Ike housing...give or take, maybe $1K here...did without flash for a bit, and then added an Ike DS-50 and slave TTL controller...got used to THIS, and added a DS-125...now saving pennies for an INON WA converter.
(again IMHO)...the Oly 50-series are used by SO many folks that there is a PLETHORA of hint & tip info found by Google-ing...other cams may be better, but none are SO documented... in most shallow OW conditions, auto-EVERYTHING will give a 25% kill ratio if sound post-process is used (my choice, PS C2 and PSE2). Today, the HEART of a UW digi-cam system is the COMPUTER...strongly consider online learning for easing the learning-curve of PhotoShop et al ... www.eclecticacademy.com is TOPS!
If ANY of y'all are in reach of N FL & are considering less camera, PLEASE visit me in the Springs and see what a few bucks more will give you...
Norm