This post is maybe a bit early as the Ixus 220 HS is still a work in progress. The WP-DC41 underwater housing for this camera holds down the menu button when the case is closed, so when you switch it on it goes into a limited functionality mode (underwater mode appears on the screen); the up, down and menu buttons are disabled. As a result we can't set iso underwater, or select 240fps mode for example.
Is this something that could be addressed with CHDK? The underwater mode has an exposure adjustment that is controlled with left right and selected with set, so maybe ISO and slow motion mode could be given similar menu slots with CHDK?
The canon underwater housings are really good; the buttons match up really well and the seals are excellent and go well beyond spec, but recently they have started to omit some of the buttons and invoke an underwater mode on some models, which rather than assisting use while underwater simply cuts out some of the functionality!