First off, with under water shots you REALLY need to be close. Water is never clear and yu need to minimize the amount of water between the subject and the camera. Ideally you'd always be less then a couple feet fro the subject and use a wade angle lens.
And clear tropical waters help with this quite a bit...but even 'crystal clear' stuff will have spectrum absorption .. reds are 99% gone by 10m depth, for example. If this wasn't snorkeling, I'd be recommending a strobe (which gets expensive, fast).
The best way to go is to buy a housing from a "point and shoot" that is intended for SCUBA diving. These housing are very robust and if you buy one for an last generation camera then yu can always replace the camera if the housing floods with a cheap $40 used camera. But you will find that a housing rated for 130 or 200 feet is VERY robust and will take abuse and not leak, certainly not at the depths a casual snorkeler would go (maybe 15 or 20 feet at most?)
Thery have been making these scuba housing fro a long time. You can buy one that fits a recently discontinued camera and then the camera is cheap enough that you can buy two and have a spare. These housing are custom fit to just ONE make and model camera, just be sure to keep sand out of the O-rings and it will last for many years. I typically wash the housing down in fresh water under a tap, maybe even clean it with a tooth brush BEFORE I open the housing.
Never let salt water dry on the camera or the housing, do a quick rinse or just dunk in a bucket of fresh water within minutes of use in salt water, It is hard to remove dry salt from a camera or housing. Most dive boats now have a big fresh water tank where people can store their cameras when not in use.
If you don't store the camera in a bucket f crash water then a million fine grains of crystalized salt will form near the seals and then when you open the seal salt dust gets inside and will damage the seal (dry salt dust is very abrasive)
This is all spot-on. I've been doing UW photography (on scuba) for the past 25 years and the general rule of thumb on camera floods is
"Not if, but WHEN".
What I'll add to the above is that when you put a camera into an UW housing,
you become the person responsible for maintaining the O-rings and their water-tight seal - - this doesn't take that much skill (or time), but it does call for discipline.
So keep the new camera in a bucket when not in use, clean it with a brush under the tap before you open it and it will last for years. I live near the beach so under water photography is something I can do every week and with care the camera seals last for years. On the other hand people who only shoot in salt water once a year on vacton don't know how to take care of the equipment and are lucky to have the gear last even a week.
Bottom line is salt water id horrible, corrosive and abrasive and requires yo to be paranoid, or rich enough to replace your camera every week.
Yes, its those salt water crystals that are tiny daggers cutting away at your O-rings - - literally. The important part is to get a
prompt freshwater rinse of your gear after it has touched saltwater, to dilute the salt that got down into the nooks&crannies (ie, your O-rings).
And as ChrisA says to leave the camera wet, we do eventually have to go home from Holiday, so the cameras will get dried out - - you can usually minimize your wet-dry-wet-dry cycles by being aware of the issue, plus tricks such as throwing a damp beach towel over the camera to limit how much it dries out.
A small point and shoot that is one generation old will cost under $100 out perform an iPhone camera. Buy one housing and two cameras and a tooth brush and a bucket.
There's aftermarket companies that make good stuff (such as Ikelite) as well as some scuba-rated and snorel-rated clear plastic housings that are sold by Canon/etc. Feel free to pick something that looks promising and ask for opinions.
-hh