Don't put ice directly in the kayak... you'll just end up with a sloshy mess as the ice reduces to an inch of cold water throughout your kayak. In a pinch, you could use a couple heavy duty contractor garbage bags. Not great, but better than nothing. The main problem is that those pargos and other Mexican fish are mean buggers... with all sorts of spines that will quickly make holes throughout your bags. Still better than nothing, I guess.
Nowadays, we bring tons of ice on boats along with fish bags(more versatile, especially for deck space and/or travel). Once in a while, we use standard ice chests or fish holds. As you know, for kayaks... it's a far different story. Kayak diving is a bit different from boat diving due to the space available.
Up here in Northern Californiai, the waters average ~50deg F. We don't last more than 5 or 6 hours in the water and it's so cold, that I don't worry too much about fish storage. In Hawaii, it's much different. While not as hot as Mexico, it did force me to rethink fish care options.
There are 3 main ways I bring ice on a kayak.
The first is for small fish. Costco has these bags available for very cheap. When I first bought it, they were 2 for $5! They were also a cheaper nylon outer material which repelled water more effectively. The price has gone up, but still an affordable option. They're not of great quality, but are excellent options for storing smaller fish (like parrotfish or pargo amarillo). Nice cheap solution. I've gone through a few, but no complaints. Here's a pic of the bag itself and an inside shot of a parrotfish stowed in my front hatch.
For a top deck fishbag, I've used this style of fishbag. The size is nice for most reef fish at 36"x20". It works okay, but the 3-sided zippers leak water quickly and the broad surface area gets hit hard by the sun. You can store it in your hatch, but the way the zippers are situated makes it cumbersome to access. The zippers wll also cause the water will leak out (not really a deal breaker, but leaking water signifies bad insulation). I've used it for halibut and a few other times for regular fish, but prefer either the first or last kayak fishbag option.
This is the premier bag, especially for kayaks with a large rear hatch like the 2-hatch scupper pros. The Da Kine tapered fish bag was made specifically for this application. This one is a 6-ft model, but the 5-ft model is probably a better fit. I slip the whole thing into the rear hatch and toss a 7# bag of ice in there. The top zipper makes access a cinch and will easily handle white sea bass, wahoo, dorado, etc. The material is very nice and heavy duty. Hard to tell from the perspective of the camera, but it tapers from a little over 2ft wide on one end to 1ft wide on the far end, fitting the shape of the kayak quite nicely. The down side? No one makes them anymore... 
I owned one of these for a bit:
http://www.creativefeathers.com/details.cfm?id=188
Worst kayak bag design... it had some thin insulation on the top and bottom, but the sides were plain nylon! The zippered was a pain as well. I promptly returned it with a handful of complaints.
Fish bags for kayaks are such a small market segment that I'm surprised Da Kine even bothered for the brief time that it did.
Sorry I sidetracked the thread so much into kayak fish bags... don't even get me started on boat fish bags. 
BTW, that Cobra Fish 'n Dive is indeed a beast of a kayak. :laughing3:
I agree it's already way heavier than it needs to be. Stable as heck though.
A small insulated bag in the forward center hatch would be a good way to keep smaller reef fish cold, but for big fish... a big deck bag would be the only real option for that kayak style.
Just shoot all your big fish at the end of your dive.