I change the CF card after every dive and I do 5 dives a day.
Wow, you change your card after every dive? Do you use a 512mb?
I use a 1 gb card and always can get at least 2 dives on a card shooting RAW (compressed D70 ~6Mb ea). I can't justify diving more than 2/day with my wife and kids there. As soon as I hit the dock I'm in full on sand-castle mode.
I agree that its a bad idea to open the housing on the boat. The way I work I don't open the housing except on land. I wouldn't want a solution that would require me to open the housing after every dive. If you want to switch lenses for a specific purpose, then you have to open the housing anyway.
I think the best solution would be to look into a larger card. Weather or not you have to open the front, back loosen screws etc. if you have to open the housing after every dive something is wrong. Unless you are on a liveaboard you don't want to deal with laptops or storage devices on the boat either. So going for a bigger card really isn't an expense over multiple smaller cards since the cost per megabyte is about the same.
Every housing has its pros and its cons--Ikelite is no different. I just think you get a lot of value out of Ikelite housings--price, TTL, security etc. I think to say that the port selection is limited is a misleading. Except for the 10D/Sigma 12-24mm, I don't think there is one critical lens missing. And even that may be corrected soon with either the Sigma 10-20 or the 8" dome port.
Besides the poster was asking about the 20D not the 10D. For this camera you have lots of choices with Ikelite ports. Sigma or Canon macro lenses 50-100mm, Sigma or Canon WA 15mm FE and Canon 10-22mm and a midrange zoom with the two Canon kit lenses. Ikelite has ports for much more, but I think that covers the critical UW list.