Gerard,
I'd like to respond directly to the question in your subject line.
I, too, have a Minolta 7xi and Ikelite housing (I even had Ike install an extra "custom" control switch so I had slightly more control of the camera), although it has been sitting in a box unused for years now. I bought the 7xi (which is a great camera, BTW) and housing primarily because I already owned several Minolta lenses, and it seemed like a "waste" to switch to a different brand that couldn't use those lenses.
I'll state this as simply as possible: Sticking with Minolta just so I could use my old lenses-- even when it was clear that the uw photography world was dominated by and producing the most gear for Nikon and Canon -- was the worst mistake I've ever made relating to photography gear. I continued to have to buy more Minolta-compatible lenses, and increasingly found that my "upgrade" options were severely limited. In order to be able to use less than $1,000 of old lenses, I ended up spending thousands of dollars going down a dead end road.
So, years later, when I decided to switch to DSLR (after using a housed Nikon Coolpix for awhile), I decided I was "starting from scratch" and was just looking for the best gear with the broadest uw photo support and maximum future upgradeability options possible. I chose Canon, but Nikon would arguably have been just as smart. Minolta (or Pentax or some other less uw-supported brand) would not have been.
If you look at what it will cost you to get a good DSLR plus essential lenses, I believe you'll find that you won't spend that much more (probably only a few hundred dollars) to switch to Canon or Nikon vs. trying to stick with Minolta (now Konica Minolta). For example, a Canon Rebel XT or 20D -- with a 10-22mm lens for wide angle and a 60mm macro lens for fish shots and macro -- will cover the vast majority of uw situations, won't set you back that much, and will have a wide range of housing options...PLUS will leave the future wide open for you.
Bite the bullet. Acknowledge your Minolta history for what it is: history. When you do decide to go digital (DSLR), don't limit yourself just because Minolta is what you "used to shoot."