In my opinion, go straight on your 7D. Once and for all if you gonna continue diving in future. Imagine, spend some money on P&S and later spend again on your 7D. You are already good with your 7D. I am like you, only 20 logged, but I shoot Nikon D90/Canon 7D on land for years. I got a used rig and it save me lot of money. Currently using Canon rebel XTI/400d in Ikelite housing with 2x DS125 strobes. 60mm macro & 10-22mm for wide angle. Just got another used ikelite 7D housing and the rest of the stuff remains. Looking forward to try the 7D housing.
-for the first dive trip, try to get familiar with viewing the tiny view finder. Imagine you are not viewing straight to your view finder now, you have housing, mask & some water in between your eye & viewfinder. I was struggling getting focus/composition at first. Try move your mask as close as possible to the housing's viewfinder will help. P&S uses live view which is pretty easier but the focus lag & shutter lag is unacceptable.
-buoyancy. I was struggling too getting buoyant while taking photos. I overcome this first by using the advantage of fast focus lock & shutter of dslr. Lock the focus first, then Framing your subject, then snap. It can happen in less than 1s. When you dive more and more with your rig, you will be familiar with your buoyant. Some might say you gotta master your buoyancy first before bringing your rig down but I find that its totally different when you dive normally & dive with your camera rig. You gotta familiar diving with your rig.
-4 divers/guide. I was the only photographer. The guide wont wait for you to take photograph as its not fair for others. I totally agreed. Try to merge with other photographer or get a private guide which cost you a little more money. Please the guide to find some subject for you, and concentrate shooting. Spend some 5-10 minutes snapping the same subject (Macro), sooner or later you will master the composition & strobe's position.
-strong current. It was very hard to snap on a strong current situation. You need to sync your kicking, buoyancy & photographing at once. Try to get your body close to the walls, or bottom. Sometimes you just gotta give up the subject in this situation, but try first. Again sooner or later you gonna master it.
-TTL. I found TTL is very helpful as I don't have to take care of the flash power. The only settings I change down there are aperture, shutter speed & strobe positioning. The other settings are all preset. On the later stage, you will need to change more setting like focus point, ISO & metering mode. You will have no problem with those setting once your familiar with your rig.
-Getting upset. I got some very annoying photos during my first dive trip with the rig. There are only a few ok-ok pictures which is long way near satisfactory. Dont get upset as you will definitely learn mistake from your first dive trip. Review your pic on each dive day and sure you will do better on your next dive day.
-Rig too negative. I found my rig is too negative (I dont know about others). It was very tiring holding the rig down there. Get a float arms or DIY something. I use 40mm thick wall PVC pipe with both end capped & cable tied on my arms. It works like a champ. My rig is now a little negative which is comfortable for me.
-dives site. Choose a macro dive sites if you like macro & vice versa. It does matter as you will have more opportunity on the subject you are looking for.
-Travel with the rig. Huge differences between travelling with P&S setup & dslr setup. If you go for dslr setup, be prepare with the weight & size.
I got some pretty satisfying pic for my 2nd dive trip as a reward of some hard work. Looking forward for my 3rd dive trip with my 7D rig. I hope my opinions & experiences are encouraging you go straight for 7D setup. Cheers.