Burial at sea is legal in the United States under an EPA “general permit,” but it must follow strict rules.

For a full body (non-cremated) burial, the EPA requires the site to be at least 3 nautical miles from land and at least 600 feet deep (some Florida/Gulf areas require 1,800 feet). The body may be placed in a weighted shroud or a properly prepared casket to ensure it sinks quickly and permanently.
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Plastic materials aren’t allowed, and the burial must be reported to the EPA within 30 days. Cremated remains can be buried at any depth, but still at least 3 nautical miles from land.

Website: https://www.va.gov/burials-memorials/eligibility/burial-at-sea

To apply for burial benefits at sea through the VA, you can follow these steps:                                                                     

Eligibility: Veterans, service members, and some family members may be eligible for burial in a VA national cemetery, including burial at sea.

Pre-Need Determination:  You can apply for a pre-need eligibility decision letter to help plan for burial in advance.

Burial Allowances:  If eligible, you may receive burial allowances to help cover costs for burial and funeral services.

Application Process:  Gather necessary documents, such as discharge records, and submit your application to the National Cemetery Scheduling Office.