I don’t think any of the cheap decent compact waterproof cameras will go to 18m. I have a Fuji XP, which is rated to 15m and I have used on plenty of dives to 12m (the max depth my son is currently allowed to dive to) I also have an Olympus TG4, which is more expensive, but again only waterproof to 15m. For the TG-4 I have a separate dive housing, which goes to around 40m, but certainly not a cheap option.
The GoPro style linked to above is probably the best bet, they seem to be getting very common underwater now a days.
And as mentioned above don’t use it for your OW dives, wait until you are confident in the water before having the camera with you.