I’m not sure what your criteria for "affordable" is but the Canon G7, G9 and the new G10 are all amazing Point-and-shoot’s with options for advanced manual settings if you so choose. Canon makes underwater housings for them as does some other company. Brand new G10 (14MP) retails for $499 but I’m sure you can find it less online. That’s the same price I bought my G7 (10MP) and G9 (12MP) for when I bought them like 3 and 1.5 years ago respectably. The housing were NOT the same between the G7 and G9 (annoying) and it looks like the G10 is just different enough that the housing will be different for it too. BTW - Housings can be had online for under $200. I’d put the actual model number into Google Shopping and see what you find.
"http://www.usa.canon.com/consumer/controller?act=ModelInfoAct&fcategoryid=144&modelid=17624"
"http://www.usa.canon.com/consumer/controller?act=ModelInfoAct&fcategoryid=144&modelid=15669"
Either model will serve you great for years to come.
Good luck and good shooting,
Eric
PS - I don’t work for Canon, just a happy customer