Sorry, but there's no such thing as cheap AND good with metal detectors. The Minelab Excaliber is surely the best UW detector on the market. I use a Whites PI pro. About half the cost of the Minelab, and I am very happy with it. But this will still run you about $600.
If your taking it into salt water than it had better be a pulse induction (PI) machine. Everything is junk in the salt enviroment. Alos, use it on land for a while before taking it under water. Learn how it responds to different objects at different depths. This can save you a lot of recovery time underwater.
Lastly, the majority of what you find will be junk. For every piece of jewelry you see in a picture of a heap of rings and jewelry recovered from the sea, there are pounds upon pounds of scrap iron and aluminum recovered that you dont see.