Apple does not offer any insurance plan that would replace a phone that is lost in a lake. It does offer AppleCare+ which covers accidental damage repair for a small fee but does not cover a lost phone. See: AppleCare+ - iPhone - Apple
AT&T does offer insurance plans that cover lost phones. See: AT&T® Official - Unsupported Browser Page
If that is what you have, they use the following site for filing a claim: http://www.phoneclaim.com/att/