iPhone replacement

If you bought AppleCare+ Theft and Loss when you bought the phone, you can file a claim to get the phone replaced for a modest fee. If you didn't, you can't and if you need to buy a new iPhone, you'll have to buy one at full cost.

Depends on whether or not you paid for a special plan such as AppleCare+, which also has a loss or theft option. However, if you don't know about it, then I doubt that you have it. There really isn't much you can do about it if you can't find it. If you could somehow fish it out, it's probably dead at this point as the water resistance (even if it holds to the IP68 standard) isn't that long. If you could manage to find it, you should be able to require a service replacement at a price that's less than new. That's about all the discount that you'd be likely to get as accident isn't covered under the warranty.

I believe it depends on the warranty on your phone and if you have Apple care. Try this link to see what your eligible for:iPhone Express Replacement - Apple Support

No.

Any kind of out of warranty replacement requires you to have your defective phone in your possession.

Apple will not give you any kind of discount because you destroyed your recently purchased property.