I shot with a Nikonos V during Caribbean dive trips back in the late '90s, but rented the rig during each dive trip. UW photography was relatively easy with the film Nikonos - since the camera was integral with the housing and external strobes were synchronized with the auto-focus shutter mechanism.
Never took any UW classes and I'm nowhere near expert status. All I wanted to do was record my underwater scuba adventures and hopefully get a few worthy shots for framing. Got quite a few decent pics and learned by reading, doing and asking questions on scuba diving web-forums.
Like you, I went digital with my land camera - about 5 years ago. After research I decided on an Olympus "Pro-sumer" digital camera. Essentially, it's a point-and-shoot, impressive automatic, but with full capabilities for manual overrides and custom exposure settings. A camera to grow with.
My plan was to initially buy the affordable Olympus housing, learn the basics and upgrade to a more durable housing. Upon learning tricks from a digital diving web forum and buying many upgrades, like digital controllers, trays, strobe arms, dual strobe units and several gigs in memory cards - the endeavor became quite expensive . . . thousands of $$$.
It seems though, from my quick scan of Amazon.com write-ups, the Canon WP-DC8 housing has good user reviews - and a great value, considering the housing costs less than 150.00. Ikelite housings start at $400., although of a noticeably higher quality. For new photographers and occasional tropical uses, it makes a lot of sense.
You will find though, that the internal flash will create what's called "backscatter" - or flash snow. This is caused by minute particles between the subject and flash that reflect the bright light - and unavoidable with a point & shoot's integrated flash due to the close proximity to the camera lens.
To avoid backscatter, seasoned UW photographers have learned to use at least one, preferably 2, digital strobe light units that are extended on aluminum adjustable arms to varying lengths - possible to several feet from the lens. This makes a huge difference in picture quality.
There is much to learn about this hobby - but be warned, it is addictive. You will want to upgrade after a few uses, but be realistic with the level of quality of your pics and decide if the investment of more gear is worthwhile.
I would recommend hanging around THESE FORUMS
for a while, I did for years and learned a great deal . . . click on Forums and feel free to post questions there. You will get way more experienced responses than here.
Ask me about anything specific though and I'll try to help you out. Remember though, the most important thing you can do is check the housing's O-rings and control seals, BEFORE sealing the housing. The greatest anxiety UW photographers have is UW flooding under pressure, by improper cautions when opening/closing a wet camera on the dive boat - to change batteries, Memory cards, or for other adjustments.