Rainbow, I agree this issue is most significant over summer months, but is worth looking into.
You've made an interesting observation re the heating of the inside of kayaks on the water and then the subsequent radiation of that heat into the fish well. As a scientist I have to ask - have you actually measured both the internal air temperature and the temperature of the external side walls of the well?
I haven't and it's something that will need to be checked in the context of the cooling/heating of the catch in the rear well.
I will make an anecdotal observation based on the Perception Swings I paddled at the time I began using insulated covers (mid 1990's). This is one of the reasons I decided they were adequate for my purposes:
When kitting up on the beach at dawn but before sunrise, with all hatches open, the Swing is at a relatively cool ambient temperature, including the internal air space. Lunch time or early afternoon was usually the first time I opened the small hatch in the cockpit because of the way I stowed my gear. My observation on opening the hatch was that it "sucked" in air rather than "blew" air out. The implies that despite having been in direct sunlight for a significant period (and with water pressure pressing against a relatively flat hull) the internal air space had cooled and shrunk, not heated and expanded. To extrapolate this further, with the reduction in internal temperature there isn't any heat to radiate into the rear well.
I can only assume at this point (without taking measurements) that heat on the topsides is counteracted by:
1. direct absorption by deck wash and spray
2. evaporation of water off the deck
3. absorbtion directly through the hull to the sea.
For this reason, to be scientifically valid, any and all temperature testing would need to occur on the water with a real catch aboard to properly indicate the net energy (heat) balance between direct radiant input and convective, evaporative, and conductive output.
The reason for using the Swing as the prime example is that this was before I started fitting accessories and these kayaks were the nearest to hemetically sealed. The Prowlers for example use a less airtight hatch system. The Profish doesn't have a hatch I open on the water.
Regarding warm sea temps entering scuppers, totally agree this will have an impact. Again this can be managed, as has been mentioned in another posting, by looking at trip duration and the timing of when fish are kept rather than released.
Regarding windage, centre of gravity changes, and load distrubution: observations by individuals can vary, and will have greater or lesser significance to those individuals depending on their operational conditions. Personally I find not having a raised rear deck advantageous
And to reiterate my closing in the above posting, I don't disagree with placing the catch directly into an ice slurry. This will certainly be the best for the catch, and for some paddlers a chillibin is the way to best acheive this. For me, again to reiterate, due to the size of some of the fish I handle and some of the conditions I operate in, I choose to use covers. They've proven over the last 15years to be perfectly adequate.
Finally, don't missinterpret this posting, I'm not trying to create an arguement, just explain my logic in continuing to use covers. I enjoy eating my catch, and so do family and friends