It sounds like you have been doing some research as you mentioned some models.
There is no need to go with a 4MP camera when there are 7 and 8 MP models available.
Compare F stop ranges, shuttter speed ranges, RAW capability (if that is important to you), manual white balance capability (a must in my opinion), macro mode distances, type of memory card and maximum size the camera can accomodate, availability of a housing, strength of the built in strobe which may require a hands on test.
Keep in mind that the larger the megapixel rating of the camera the larger the file size of the pictures. That translates to needing a larger memory card.
Do not consider the capability of digital zoom only optical zoom. Digital zoom is basically worthless.
As far as use on land and underwater, a 10X optical zoom is great for land use but of little or no use underwater. A 3 or 4X optical zoom is all you need for underwater use. So you have to decide if a 10x optical zoom camera has all of the other features needed for underwater.
You may start out using the camera on Auto but will eventually switch to Manual controls for better quality photos.
Ikelite makes under $300. housings for some of the point and shoot models. Compare that to the camera brand housings as the price difference may be close enough that you would want the Ikelite housing.